[POWERPC] Allow for bootwrapper utilities being in different directory to objects

It's possible that the executables which are built as helpers for the
bootwrapper stuff might end up in a different place to the intermediate
object files. Handle that.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>