Add documentation for rootfs creation and installation

[hansemro]

It would be nice to have some documentation on building a simple rootfs with busybox. Once more devices become accessible via device tree, I will look into buildroot and postmarketOS.

[mj-sakellaropoulos]

I have buildroot working on Ubuntu 20.10
Download and untar : https://buildroot.org/downloads/buildroot-2021.02.1.tar.gz

# need to install various dependencies, for xconfig you need qt5
cd buildroot
make xconfig

• Under Target options, select ARM, cortex-A9, enabled SIMD and VFP, EABIHF, VFPv3
• Under toolchain, you can specify a custom toolchain or have it build one from scratch
• Under Kernel, you can pass it your defconfig from out of tree
• For the devicetree, if it doesn't handle out of tree .dts properly you can manualy put it in output/build/linux-{version}/arch/arm/boot/dts/
• Under target packages, you can include the packages you want
• Under Filesystem images, you can choose to cpio the rootfs for convenient loading as ramdisk via fastboot

minimal buildroot config : https://gist.github.com/mj-sakellaropoulos/25342160a6d9b66851c9108624850518

[hansemro]

Neat!

[hansemro]

398b51c: Added steps to create and install busybox rootfs.

I will look into buildroot next.