main/postmarketos-mkinitfs: Support initramfs compressions other than gzip

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_initramfs_compression='lzma' or even deviceinfo_initramfs_compression='lzma -9'

When no option is specified it defaults to 'gzip -1'.

I'm not very experienced in shell scripting so I'm not sure if this is the best way to do it. I've also added dependencies to compression utilities for all possible initramfs compression algorithms.