Makefile: support optional build_id for contents.xml generation

Add support for passing an optional `build_id` argument to the Makefile,
which in turn used by gen_contents.py to add unique build_id into the
generated `contents.xml`, as required by Axiom.

Signed-off-by: Viswanath Kraleti <[email protected]>

Author: vkraleti

Makefile

@@ -19,7 +19,7 @@ all: $(PLATFORMS) $(PARTITIONS_XML) $(CONTENTS_XML)
 	$(TOPDIR)/gen_partition.py -i $^ -o $@
 
 %/contents.xml: %/partitions.xml %/contents.xml.in
-	$(TOPDIR)/gen_contents.py -p $< -t $@.in -o $@
+	$(TOPDIR)/gen_contents.py -p $< -t $@.in -o $@ $${BUILD_ID:+ -b $(BUILD_ID)}
 
 lint:
 	# W605: invalid escape sequence