Merge branch 'add-update-rule-to-makefile' into 'master'

Add a rule to make updating submodules more convenient

See merge request Librem5/developer.puri.sm!248

Makefile

@@ -232,3 +232,8 @@ dummy:
 check:
 	xmllint --noout $(XML_SOURCES)
 	flake8
+
+.PHONY: update
+update:
+	git submodule init
+	git submodule update