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>
diff --git a/post/Makefile b/post/Makefile
index 20a463a..2fa6f8a 100644
--- a/post/Makefile
+++ b/post/Makefile
@@ -10,9 +10,9 @@
 
 obj-y += drivers/
 obj-$(CONFIG_PPC) += lib_powerpc/
-ifneq ($(filter mpc83xx mpc8xx ppc4xx,$(CPU)),)
-obj-y += cpu/$(CPU)/
-endif
+obj-$(CONFIG_MPC83xx) += cpu/mpc83xx/
+obj-$(CONFIG_8xx) += cpu/mpc8xx/
+obj-$(CONFIG_4xx) += cpu/ppc4xx/
 ifneq ($(filter lwmon lwmon5 netta pdm360ng,$(BOARD)),)
 obj-y += board/$(BOARD)/
 endif