update-kernel: fails with modinfo returning wildcarded firmware names

Summarizing a long thread aports#13302 (comment 259618), and landing resulting core issue where it should pertain, so that it does not fall aside.

While building modloop, firmware names extracted from modinfo -F firmware may contain $FW names with wildcards.
update-kernel then does not process these as if [ -e "$ROOT/lib/firmware/$FW" ] statement fails with * wildcards.

An sample real-life consequence of this is Alpine failing to incorporate PiZero2W firmware file within Releases modloop.
This may silently happen too for some other drivers...