UBUNTU: [Debian] basic hook support

Add the infrastructure for Makefile macro overrides. This
is primarily used by LTS backport branches to supersede master
branch settings such as do_tools_common, etc.
Signed-off-by: Andy Whitcroft <apw@canonical.com>

debian/rules

@@ -26,6 +26,10 @@ include $(DROOT)/rules.d/0-common-vars.mk
 # Pull in some arch specific stuff
 -include $(DEBIAN)/rules.d/$(arch).mk
 
+# Pull in some branch specific stuff. Used by LTS backport
+# branches to override master branch settings such as do_tools_common.
+-include $(DEBIAN)/rules.d/hooks.mk
+
 # Maintainer targets
 include $(DROOT)/rules.d/1-maintainer.mk