Build: introduce ${BUILD_PLAT} target to create the top build directory

Some platforms (for ex. UniPhier) want to create files in the very
top of the build directory.  Add ${BUILD_PLAT} so such files can
depend on it.

Make existing directory targets depend on ${BUILD_PLAT} because
they are sub-directories of ${BUILD_PLAT}.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
3 files changed