This guide explains step by step how to create a minimal linux image for the Rasperry PI 3, using buildroot and a Linux host system (such as Fedora, Debian or Ubuntu).

The kernel is armv8 (64-bits).

Instead of using glibc, we will use uCLibc.

We will use a small initramfs image.

All files are stored in a single fat32 partition of the uSD card.

This step describes how to download buildroot, and build an image.

Create a working directory on the Linux host PC, for instance:

~$ mkdir -p lfs/rpi3/min_img
~$ cd lfs/rpi3/min_img
~/lfs/rpi3/min_img$

Download and extract the latest released version of buildroot:

~/lfs/rpi2/min_img$ wget --no-check-certificate https://buildroot.org/downloads/buildroot-2017.11.tar.bz2
~/lfs/rpi2/min_img$ tar -f buildroot-2017.11.tar.bz2 -x
~/lfs/rpi2/min_img$ cd buildroot-2017.11/
~/lfs/rpi2/min_img/buildroot-2017.11/$

Use either make config or make xconfig to adjust the configuration.
The configuration is:

Target options

Target Architecture: AARCH64 (little endian)

Build options

Enable compiler cache

Toolchain

Custom kernel headers series4.9.x

System configuration

Root password: root (or any password you want)

Kernel

Kernel versionCustom Git repository
URL of custom repositoryhttps://github.com/raspberrypi/linux.git
Custom repository versionrpi-4.9.y
Defconfig namebcmrpi3
Select: Build a Device Tree Blob (DTB)
Device Tree Source file namesbroadcom/bcm2710-rpi-3-b

Target packages

Hardware handling / Firmware - Select: rpi-firmware

Filesystem images

Select: cpio the root filesystem (for use as an initial RAM filesystem)
Compression method: gzip

Host utilities

Select: host dosfstools
Select: host genimage
Select: host mtools

Notes:

By default, the build will use all cpu cores. Specify the maximum number of cores you want to use (For instance, my machine at home is 8-cores CPU, I usually use 4 or 6 cores for the build)

You can download the configuration file rpi3_minimal_defconfig

If you want to use the configuration file instead of changing the configuration manuall, you can use these steps:

~/lfs/rpi3/min_img/buildroot-2017.11$ wget https://rawgit.com/osingla/LinuxFromScratch/gh-pages/rpi3/minimal_img/rpi3_minimal_defconfig ~/lfs/rpi3/min_img/buildroot-2017.11$ mv rpi3_minimal_defconfig configs/ ~/lfs/rpi3/min_img/buildroot-2017.11$ make rpi3_minimal_defconfig

Depending of the speed of your build machine and your Internet speed, the build could take a while, typically in the 20 to 30 minutes for a first time build.
Build now:

~/lfs/rpi3/min_img/buildroot-2017.11$ make

Once the build is done, you will have the kernel, the Device Tree Blob (DTB), and the root filesystem tar file:

This step describes how to create a uSD card using the image previously built.

Any uSD card will do, assuming it's at least a 2 GB card. >br>
We will create one partition on the uSD card. Actually, what really matters is the first partition of the uSD card:

the 1st partition needs to be FAT32, and
the 1st partition needs to be at least 128 MB

The first fat32 partition will hold

the firmware (boot loader)
the kernel and the device tree blob (dtb)
the initramfs image

Use fdisk to create the partitions:

partition 1 : type fat32 (12, 0x0C), 128 M

Once you have created the partitions on the uSD card, assuming your linux host assigned the device /dev/sdg (you can check using dmesg), you will do something like this to copy the firmware, the kernel and the DTB:

~/lfs/rpi3/min_img/buildroot-2017.02.8$ rm -rf /tmp/rpi3-sd
~/lfs/rpi3/min_img/buildroot-2017.02.8$ mkdir -p /tmp/rpi3-sd
~/lfs/rpi3/min_img/buildroot-2017.02.8$ fdisk -l /dev/sdg
Disk /dev/sdg: 62.5 GiB, 67108864000 bytes, 131072000 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device Boot Start End Sectors Size Id Type
/dev/sdg1 * 2048 131071999 131069952 62.5G c W95 FAT32 (LBA)
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo partprobe /dev/sdg
~/lfs/rpi3/min_img/buildroot-2017.02.8$ mkfs.vfat -n rpi3-sd /dev/sdg1
mkfs.fat 4.1 (2017-01-24)
mkfs.fat: warning - lowercase labels might not work properly with DOS or Windows
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo mount /dev/sdg1 /tmp/rpi3-sd
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo cp -r output/images/rpi-firmware/* /tmp/rpi3-sd/
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo cp output/images/bcm2710-rpi-3-b.dtb /tmp/rpi3-sd/
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo cp output/images/Image /tmp/rpi3-sd/
~/lfs/rpi3/min_img/buildroot-2017.02.8$ sudo cp output/images/rootfs.cpio.gz /tmp/rpi3-sd/

You now need to edit two files on the uSD card:

cmdline.txt
config.txt

Since we use an initramfs image, we need to change the kernel parameters specified in /tmp/rpid3-sd/cmdline.txt:

root=/dev/mmcblk0p2 rootwait console=tty1 console=ttyAMA0,115200

console=tty1 console=ttyAMA0,11520

We need now to make a few modifications in /tmp/rpid3-sd/config.txt

kernel=zImage

kernel=Image

#initramfs rootfs.cpio.gz

initramfs rootfs.cpio.gz

arm_control=0x200

You can now unmount the uSD card and eject it:

~/lfs/rpi3/min_img/buildroot-2017.11$ sudo umount /tmp/rpi3-sd
~/lfs/rpi3/min_img/buildroot-2017.11$ sudo eject /dev/sdg

I wrote a simple shell script which create the two partitions on the uSD card, and then copy the files.
Feel free to use it instead of typing all the commmands above.

~/lfs/rpi3/min_img/buildroot-2015.11.1$ wget https://raw.githubusercontent.com/osingla/LinuxFromScratch/gh-pages/create-sdcard1.sh
~/lfs/rpi4/min_img/buildroot-2015.11.1$ ./create-sdcard1.sh --device /dev/sdg

YOU NEED TO ADJUST THE DEVICE NAME.
PLEASE TAKE A LOOK AT THE OUTPUT OF dmesg TO SEE THE DEVICE NAME.

You are now ready to test your uSD card.

Insert this card on your Rasperry PI3, and apply power.
If all goes well, you should get a shell.