uniphier: make sure to create build directory before ROT key

Building the UniPhier platform in parallel with TRUSTED_BOARD_BOOT=1
could fail due to non-existing directory.  It might be difficult to
reproduce, but here is an easier way to trigger the problem:

$ make PLAT=uniphier TRUSTED_BOARD_BOOT=1 MBEDTLS_DIR=mbedtls certificates
  OPENSSL build/uniphier/release/rot_key.pem
/bin/sh: 1: cannot create build/uniphier/release/rot_key.pem: Directory nonexistent
make: *** [build/uniphier/release/rot_key.pem] Error 2

The $(ROT_KEY) must depend on $(BUILD_PLAT) so that the build directory
is created before the key.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

plat/socionext/uniphier/platform.mk

@@ -101,7 +101,7 @@ $(BUILD_PLAT)/bl1/uniphier_rotpk.o: $(ROTPK_HASH)
 $(BUILD_PLAT)/bl2/uniphier_rotpk.o: $(ROTPK_HASH)
 
 certificates: $(ROT_KEY)
-$(ROT_KEY):
+$(ROT_KEY): | $(BUILD_PLAT)
 	@echo "  OPENSSL $@"
 	$(Q)openssl genrsa 2048 > $@ 2>/dev/null