This replaces the mkinitfs script with a re-write (in Go). The
re-written mkinitfs will atomically replace the initfs in /boot, check
for free space before doing so, and try to do whatever it can to not
leave the system in an unbootable state by botching the initfs
creation/installation.
pmb:cross-native with go cross compiling doesn't work exactly, it makes
the correct binary, but on Alpine go uses -buildmode=pie which creates a
dynamic thing, and the interpreter is wrong (e.g. it it set to use the
host arch's interpreter, like /lib/ld-musl-x86_64 even though
GOARCH=arm64)
The init.sh script is no longer a template, "initramfs-extra" is used.
Fixes https://gitlab.com/postmarketOS/pmaports/-/issues/1019
fixes https://gitlab.com/postmarketOS/pmaports/-/issues/660
