fix passing explicit stage files

fix passing explicit stage files

This fixes installboot regress on powerpc64.

The exact same diff already landed for macppc;  efi also has the same fix
for md_init() but without the string handling cleanup that entails.

macppc_installboot.c r1.6 "Fix passing explicit stage files":

	Using `stage1' leads to a bit more cleanup since early MI installboot.c
	handles `-r', i.e. write_filesystem() no longer has needs to do the
	fileprefix() dance itself.

OK gkoehler