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
30e3cd40aeb04aa5052fa9acfc74abe72d41fa0f 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>