For historic reasons we had defined some additional PLATFORM_CPPFLAGS like:

PLATFORM_CPPFLAGS += -DCONFIG_MPC86xx=1
PLATFORM_CPPFLAGS += -DCONFIG_MPC8641=1

However these are all captured in the config.h and thus redudant.  Also
moved common 86xx flags into cpu/mpc86xx/config.mk.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>