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>

api/Makefile

@@ -6,4 +6,4 @@
 
 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
+obj-$(CONFIG_PPC) += api_platform-powerpc.o