Makefile: descend into subdirectories only when CONFIG_API is defined

All objects under api/ and examples/api/ directories are selected
by CONFIG_API.
So we can move CONFIG_API switch to the top Makefile.

In order to use CONFIG_API, the definition of SUBDIR_EXAMPLES-y
must be moved after "sinlude $(obj)include/autoconf.mk".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/api/Makefile b/api/Makefile
index fb130ff..0f85a0b 100644
--- a/api/Makefile
+++ b/api/Makefile
@@ -4,5 +4,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_API) += api.o api_display.o api_net.o api_storage.o \
-		       api_platform-$(ARCH).o
+obj-y += api.o api_display.o api_net.o api_storage.o
+obj-$(CONFIG_ARM) += api_platform-arm.o
+obj-$(CONFIG_PPC) += api_platform-powerpc.c