post: convert makefiles to Kbuild style

This commit also deletes post/rules.mk,
which in not necessary any more.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/post/board/netta/Makefile b/post/board/netta/Makefile
index 2d73f55..cd27a5c 100644
--- a/post/board/netta/Makefile
+++ b/post/board/netta/Makefile
@@ -4,10 +4,7 @@
 #
 # SPDX-License-Identifier:	GPL-2.0+
 #
-include $(OBJTREE)/include/autoconf.mk
 
-LIB	= libpostnetta.o
+CPPFLAGS += -I$(TOPDIR)
 
-COBJS-$(CONFIG_HAS_POST)	+= codec.o dsp.o
-
-include $(TOPDIR)/post/rules.mk
+obj-$(CONFIG_HAS_POST)	+= codec.o dsp.o