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>
diff --git a/Makefile b/Makefile
index b32b417..a648d8c 100644
--- a/Makefile
+++ b/Makefile
@@ -260,6 +260,8 @@
 
 include ${PLAT_MAKEFILE_FULL}
 
+$(eval $(call MAKE_PREREQ_DIR,${BUILD_PLAT}))
+
 # Platform compatibility is not supported in AArch32
 ifneq (${ARCH},aarch32)
 # If the platform has not defined ENABLE_PLAT_COMPAT, then enable it by default