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>