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/Makefile b/Makefile
index a0e5973..ca3c5c3 100644
--- a/Makefile
+++ b/Makefile
@@ -213,15 +213,11 @@
 #########################################################################
 # U-Boot objects....order is important (i.e. start must be first)
 
-OBJS  = $(CPUDIR)/start.o
-ifeq ($(CPU),ppc4xx)
-OBJS += $(CPUDIR)/resetvec.o
-endif
-ifeq ($(CPU),mpc85xx)
-OBJS += $(CPUDIR)/resetvec.o
-endif
+head-y := $(CPUDIR)/start.o
+head-$(CONFIG_4xx) += arch/powerpc/cpu/ppc4xx/resetvec.o
+head-$(CONFIG_MPC85xx) += arch/powerpc/cpu/mpc85xx/resetvec.o
 
-OBJS := $(addprefix $(obj),$(OBJS))
+OBJS := $(addprefix $(obj),$(head-y))
 
 HAVE_VENDOR_COMMON_LIB = $(if $(wildcard board/$(VENDOR)/common/Makefile),y,n)