POST: move CONFIG_POST to Makefiles

Introduce the new logical option CONFIG_HAS_POST which is set when the
platform has CONFIG_POST set. Use CONFIG_HAS_POST in the post/ Makefiles
to determine should the POST libs be compiled for the selected target
platform, or not.

To avoid breaking u-boot linking process, the empty post/libpost.a file is
created for platforms which do not have POSTs.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/post/cpu/ppc4xx/Makefile b/post/cpu/ppc4xx/Makefile
index e3f44b7..7b13413 100644
--- a/post/cpu/ppc4xx/Makefile
+++ b/post/cpu/ppc4xx/Makefile
@@ -20,10 +20,11 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 # MA 02111-1307 USA
 #
+include $(TOPDIR)/include/autoconf.mk
 
 LIB	= libpostppc4xx.a
 
-AOBJS   = cache_4xx.o
-COBJS	= cache.o denali_ecc.o ether.o fpu.o spr.o uart.o watchdog.o
+AOBJS-$(CONFIG_HAS_POST)	+= cache_4xx.o
+COBJS-$(CONFIG_HAS_POST)	+= cache.o denali_ecc.o ether.o fpu.o spr.o uart.o watchdog.o
 
 include $(TOPDIR)/post/rules.mk