Makefile: Select objects by CONFIG_ rather than $(ARCH) or $(CPU)

Convert like follows:

 CPU mpc83xx  -> CONFIG_MPC83xx
 CPU mpc85xx  -> CONFIG_MPC85xx
 CPU mpc86xx  -> CONFIG_MPC86xx
 CPU mpc5xxx  -> CONFIG_MPC5xxx
 CPU mpc8xx   -> CONFIG_8xx
 CPU mpc8260  -> CONFIG_8260
 CPU ppc4xx   -> CONFIG_4xx
 CPU x86      -> CONFIG_X86
 ARCH x86     -> CONFIG_X86
 ARCH powerpc -> CONFIG_PPC

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>