Makefile: support descending down to subdirectories

This patch tweaks scripts/Makefile.build to allow
the build system to descend into subdirectories like Kbuild.

To use this feature, use "obj-y += foo/" syntax.

Example:
    obj-$(CONFIG_FOO) += foo/

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>