Restore r332345 and r332346 from head, merged to stable/11 as
part of r333006, which was reverted in r334735.

r332345 fixes makefs(8) invocation after head revision r331843,
where makefs(8) was updated to be in sync with NetBSD.

r332346 fixes the $bootable variable position so the platformid
option is correctly applied.

This is a direct commit to releng/11.2, as these two revisions
were part of a total of four revisions merge to stable/11 (at
the time) in r333006.

Approved by: re (bdrewery)
Sponsored by: The FreeBSD Foundation