kbuild: refactor Makefile and spl/Makefile more

This commit refactors rules of directory descending
and defines u-boot-dirs and u-boot-all-dirs.
(We will need u-boot-all-dirs when using
scripts/Makefile.clean)

Additionally, rename LIBS-y to libs-y.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>