post: descend only when CONFIG_HAS_POST is defined

All objects under post/ directory are enabled by CONFIG_HAS_POST.
(post/tests.o is enabled by CONFIG_POST_STD_LIST.
But CONFIG_POST_STD_LIST depends on CONFIG_HAS_POST.)

We can move CONFIG_HAS_POST switch to the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index b20a77a..468b1bc 100644
--- a/Makefile
+++ b/Makefile
@@ -275,7 +275,7 @@
 LIBS-y += common/
 LIBS-y += lib/libfdt/
 LIBS-$(CONFIG_API) += api/
-LIBS-y += post/
+LIBS-$(CONFIG_HAS_POST) += post/
 LIBS-y += test/
 
 ifneq (,$(filter $(SOC), mx25 mx27 mx5 mx6 mx31 mx35 mxs vf610))