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/post/board/netta/Makefile b/post/board/netta/Makefile
index 5c37f49..8fc1945 100644
--- a/post/board/netta/Makefile
+++ b/post/board/netta/Makefile
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_HAS_POST)	+= codec.o dsp.o
+obj-y += codec.o dsp.o