Split up initramfs to make it smaller

Created by: ollieparanoid

The idea is to have a small "real"/"small" initramfs, and a "bigger" initramfs file on the boot partition, that gets loaded from the "small" initramfs. The big one can contain more binaries, such as resize2fs, parted, cryptsetup and an on screen keyboard. Here are the TODOs, how it could be done. It will be some effort though!

• generate a second initramfs file in postmarketos-mkinitfs, which is also compressed
• write a new init.sh script for the little initramfs: ** find the boot partition (with subpartition support, use kpartx like done now) ** mount it ** extract the second initramfs and pass control (e.g. like done here, but from a file, not from a partition)
• only include the following files in the small initramfs: busybox, kpartx

From there on we could optimize it further (compile minimal busybox, statically link busybox and kpartx against musl).