Build: rename FIP_ADD_IMG to TOOL_ADD_IMG

Now FIP_ADD_IMG takes care of both fiptool and cert_create
symmetrically.  Rename it so that it matches the behavior.

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 4232a44..4f311c3 100644
--- a/plat/arm/css/common/css_common.mk
+++ b/plat/arm/css/common/css_common.mk
@@ -49,7 +49,7 @@
 ifeq (${CSS_LOAD_SCP_IMAGES},1)
   NEED_SCP_BL2 := yes
   ifneq (${TRUSTED_BOARD_BOOT},0)
-    $(eval $(call FIP_ADD_IMG,SCP_BL2U,--scp-fwu-cfg,FWU_))
+    $(eval $(call TOOL_ADD_IMG,SCP_BL2U,--scp-fwu-cfg,FWU_))
   endif
 
   ifeq (${CSS_USE_SCMI_SDS_DRIVER},1)