fix passing explicit stage files

fix passing explicit stage files

This fixes installboot regress on octeon; same diff as
macppc_installboot.c r1.6, powerpc64_installboot r1.7 and
octeon_installboot r1.8.

loongson was the last architecture requiring this fix.  I don't have a
machine to test it myself (loongson isn't built anymore, anyway) but given
the same diff works on four other architectures, this should just work.