Build: change the first parameter of TOOL_ADD_IMG to lowercase

In the next commit, I need the image name in lowercase because
output files are generally named in lowercase.

Unfortunately, TOOL_ADD_IMG takes the first argument in uppercase
since we generally use uppercase Make variables.

make_helpers/build_macros.mk provides 'uppercase' macro to convert
a string into uppercase, but 'lowercase' does not exist.  We can
implement it if we like, but it would be more straightforward to
change the argument of TOOL_ADD_IMG.

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 4f311c3..cfbb3e9 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 TOOL_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)