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/drivers/Makefile b/post/drivers/Makefile
index cb2f1de..0b6cdf5 100644
--- a/post/drivers/Makefile
+++ b/post/drivers/Makefile
@@ -20,12 +20,10 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 # MA 02111-1307 USA
 #
-
-
-SUBDIRS =
+include $(TOPDIR)/config.mk
 
 LIB	= libpostdrivers.a
 
-COBJS	= i2c.o memory.o rtc.o
+COBJS-$(CONFIG_HAS_POST)	+= i2c.o memory.o rtc.o
 
 include $(TOPDIR)/post/rules.mk