support both compressed and uncompressed filenames for firmware in /lib/firmware

This is to help with supporting compressed firmware in pmOS, and it should be a relatively simple change in mkinitfs when compared to the much larger change required in pmaports/device packages to support compressed firmware filenames if mkinitfs didn't handle this.

In other words, mkinitfs (in the "file" hook) can search for /lib/firmware/whatever.ztd (and others?) if /lib/firmware/whatever isn't found.

Also see: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/60282

cc @Newbyte