powerpc/boot: Properly handle the base "of" boot wrapper

powerpc/boot: Properly handle the base "of" boot wrapper

The wrapper script needs an explicit rule for the "of" boot
wrapper (generic wrapper, similar to pseries). Before
7249c50fb6b570f5d11f1d37b37e1dd0591f722d it was hanlded
implicitly by the statement:

platformo=$object/"$platform".o

But now that epapr.o needs to be added, that doesn't work
and an explicit rule must be added.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>