Add support to link an external lib with ARM TF

This patch defines the variable `LDLIBS` which allows external
libraries to be specified to 'ld' to enable it to link the
libraries.

Change-Id: I02a490eca1074063d00153ccb0ee974ef8859a0e
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

make_helpers/build_macros.mk

@@ -314,7 +314,7 @@ else
 		$$(CC) $$(TF_CFLAGS) $$(CFLAGS) -xc -c - -o $(BUILD_DIR)/build_message.o
 endif
 	$$(Q)$$(LD) -o $$@ $$(TF_LDFLAGS) $$(LDFLAGS) -Map=$(MAPFILE) \
-		--script $(LINKERFILE) $(BUILD_DIR)/build_message.o $(OBJS)
+		--script $(LINKERFILE) $(BUILD_DIR)/build_message.o $(OBJS) $(LDLIBS)
 
 $(DUMP): $(ELF)
 	@echo "  OD      $$@"