postmarketos-mkinitfs: Support initramfs compressions other than gzip (!334)

The postmarketos initramfs is larger than android initramfs. This causes
problems on some devices like htc-ace that have a very small boot
partition (4MB in htc-ace). I've been able to make the boot image small
enough to fit by compressing initramfs using lzma.
With this change it should be possible to change the compression in
deviceinfo like this: deviceinfo_initfs_compression='lzma'
or even deviceinfo_initfs_compression='lzma -9'. When no option is
specified it defaults to 'gzip -1'.

I've also added dependencies to compression utilities for all possible
initramfs compression algorithms.