mkroot - Build simple bootable Linux system.

This project uses a cross compiler toolchain (usually musl-cross-make) to
build a toybox-based root filesystem and kernel that boot under qemu
(or on real hardware if available).

The tl;dr is you can go:

  $ CROSS_COMPILE=prefix- ./mkroot.sh dropbear kernel
  $ cd output/$ARCH
  $ ./qemu-$ARCH.sh

And it'll boot an emulated system under QEMU.

Running ./mkroot.sh with no arguments and no $CROSS_COMPILE environment
variable set builds a chroot for the host.

  $ ./mkroot.sh
  $ sudo chroot output/host/root /init
  $ ls -l
  $ exit

This downloads toybox into the "packages" directory, builds a minimal root
filesystem, and packages it up into a cpio file (ls output/*/*.cpio.gz)
suitable for use as an initramfs.

You can build additional things from the "module" directory by listing
them on the command line, ala:

  $ ./mkroot.sh dropbear distcc

Use "-n" as the first argument if you're adding to an existing root filesystem
you don't want to delete and rebuild.

To build for a different target, specify a cross compiler prefix using
the CROSS_COMPILE environent variable, ala:

  $ CROSS_COMPILE=sh4-linux-musl- ./mkroot.sh

The project is developed against musl-cross-make.
https://github.com/richfelker/musl-cross-make project. Running the included
mcm-buildall.sh script in that project's directory builds cross and native
compilers for all supported targets (which takes a while).

  $ cd ~
  $ git clone https://github.com/richfelker/musl-cross-make mcm
  $ cd mcm
  $ ~/mkroot/mcm-buildall.sh
  $ cd ~/mkroot
  $ CROSS_COMPILE=~/mcm/output/sh4-linux-musl-cross/bin/sh4-linux-musl- \
    ./mkroot.sh kernel

The "kernel" module builds a bootable Linux kernel for the target (it
recognizes all the musl-cross-make prefixes, for others you may have to
set the environment variable $ARCH to the right type).

  $ CROSS_COMPILE=armv5l-linux-musl-
  $ PATH=~/mcm/output/${CROSS_COMPILE}cross/bin:"$PATH"
  $ ./mkroot.sh dropbear kernel
  $ cd output/armv5l
  $ ./qemu-armv5l.sh
  $ ls -l
  $ cat /proc/cpuinfo 
  $ exit