The wrapper script needs an explicit rule for the "of" boot
wrapper (generic wrapper, similar to pseries). Before
0c9fa291 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>