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/Makefile b/Makefile
index a9618f5..e32ba19 100644
--- a/Makefile
+++ b/Makefile
@@ -598,17 +598,17 @@
 FIP_BL2_ARGS := tb-fw
 endif
 
-$(if ${BL2}, $(eval $(call TOOL_ADD_IMG,BL2,--${FIP_BL2_ARGS})),\
+$(if ${BL2}, $(eval $(call TOOL_ADD_IMG,bl2,--${FIP_BL2_ARGS})),\
 	$(eval $(call MAKE_BL,2,${FIP_BL2_ARGS})))
 endif
 
 ifeq (${NEED_SCP_BL2},yes)
-$(eval $(call TOOL_ADD_IMG,SCP_BL2,--scp-fw))
+$(eval $(call TOOL_ADD_IMG,scp_bl2,--scp-fw))
 endif
 
 ifeq (${NEED_BL31},yes)
 BL31_SOURCES += ${SPD_SOURCES}
-$(if ${BL31}, $(eval $(call TOOL_ADD_IMG,BL31,--soc-fw)),\
+$(if ${BL31}, $(eval $(call TOOL_ADD_IMG,bl31,--soc-fw)),\
 	$(eval $(call MAKE_BL,31,soc-fw)))
 endif
 
@@ -620,16 +620,16 @@
 BUILD_BL32 := $(if $(BL32),,$(if $(BL32_SOURCES),1))
 
 $(if ${BUILD_BL32}, $(eval $(call MAKE_BL,32,tos-fw)),\
-	$(eval $(call TOOL_ADD_IMG,BL32,--tos-fw)))
+	$(eval $(call TOOL_ADD_IMG,bl32,--tos-fw)))
 endif
 
 # Add the BL33 image if required by the platform
 ifeq (${NEED_BL33},yes)
-$(eval $(call TOOL_ADD_IMG,BL33,--nt-fw))
+$(eval $(call TOOL_ADD_IMG,bl33,--nt-fw))
 endif
 
 ifeq (${NEED_BL2U},yes)
-$(if ${BL2U}, $(eval $(call TOOL_ADD_IMG,BL2U,--ap-fwu-cfg,FWU_)),\
+$(if ${BL2U}, $(eval $(call TOOL_ADD_IMG,bl2u,--ap-fwu-cfg,FWU_)),\
 	$(eval $(call MAKE_BL,2u,ap-fwu-cfg,FWU_)))
 endif