main/postmarketos-mkinitfs: add /lib/firmware to list of trigger dirs

In many cases, when fw is installed in /lib/firmware, some of it ends up in the initramfs. In those situations, mkinitfs should run so that the initramfs gets the latest fw. This will result in mkinitfs possibly running in cases where it doesn't matter (i.e. device has fw in /lib/firmware but none is used in the initramfs), but I think the trade-off is worth it.

This also breaks the triggers= line into multiple lines so that it's easier to review/maintain. We can't prepend whitespace on these lines since the apkbuild parser expects dirs to be separated by ":"