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
0c9fa29149d3726e14262aeb0c8461a948cc9d56 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>