build(bl2): deduplicate sources

Deduplicating sources prevents the build system from complaining about
multiply-compiled files, which can happen if multiple makefiles depend
on a component. This already occurs for BL31.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: Ic9e67932550f07cb9e4d199f68bc46c33a611748

Makefile

@@ -1195,6 +1195,8 @@ ifeq (${BL2_AT_EL3}, 0)
 FIP_BL2_ARGS := tb-fw
 endif
 
+BL2_SOURCES := $(sort ${BL2_SOURCES})
+
 $(if ${BL2}, $(eval $(call TOOL_ADD_IMG,bl2,--${FIP_BL2_ARGS})),\
 	$(eval $(call MAKE_BL,bl2,${FIP_BL2_ARGS})))
 endif