Build: add generic way to include SCP_BL2 into FIP image

If SCP_BL2 is passed in from the command line, it is recognized by
make_helpers/tbbr/tbbr_tools.mk, and the cert_create tool generates
the corresponding key and content certificates.

On the other hand, the top-level Makefile does not care SCP_BL2, so
the --scp-fw option is not passed to the fiptool.  As far as I see
plat/arm/css/common/css_common.mk, it looks like a platform's job to
add $(eval $(call FIP_ADD_IMG,SCP_BL2,--scp-fw)).

We can make the top-level Makefile kind enough to take care of it.
This is useful when we want to have optional SCP_BL2 firmware.

Adjust css_common.mk so that Juno still requires SCP_BL2 by default.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/plat/arm/css/common/css_common.mk b/plat/arm/css/common/css_common.mk
index 86ba6df..7829e8b 100644
--- a/plat/arm/css/common/css_common.mk
+++ b/plat/arm/css/common/css_common.mk
@@ -64,7 +64,7 @@
 $(eval $(call add_define,CSS_LOAD_SCP_IMAGES))
 
 ifeq (${CSS_LOAD_SCP_IMAGES},1)
-  $(eval $(call FIP_ADD_IMG,SCP_BL2,--scp-fw))
+  NEED_SCP_BL2 := yes
   ifneq (${TRUSTED_BOARD_BOOT},0)
     $(eval $(call FWU_FIP_ADD_IMG,SCP_BL2U,--scp-fwu-cfg))
   endif