support lzma for initramfs compression too

Right now only gzip is supported. This is fine for almost all devices, except for the htc-ace:

$ git grep deviceinfo_initfs_compression
device/testing/device-htc-ace/deviceinfo:deviceinfo_initfs_compression="lzma"
main/postmarketos-mkinitfs/mkinitfs_functions.sh:deviceinfo_initfs_compression=""
main/postmarketos-mkinitfs/mkinitfs_functions.sh:       [ -z "$deviceinfo_initfs_compression" ] && deviceinfo_initfs_compression='gzip -1'
main/postmarketos-mkinitfs/mkinitfs_functions.sh:               | $deviceinfo_initfs_compression > "$2"

This one uses lzma, and for that device the feature was added in https://gitlab.com/postmarketOS/pmaports/-/merge_requests/334.

Now this device port has not been updated for the last 2 years, it's unlikely that somebody is using this until it's resolved. So this doesn't need to block introducing the new mkinitfs.

TODO:

• add lzma support
• update deviceinfo reference with a list of possible values (empty, gzip, lzma?)
• pmaports CI check that ensures we only use allowed values