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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>