fw_stub: fix -Wunused-but-set-variable for firmware files

In case we are only embedding a single firmware image the variable
"parent" gets set but never used. Add checks for the number of files
for it and only print it out if we are exceeding the single file count.
This fixes -Wunused-but-set-variable warnings for the majority of
firmware files in the tree.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks