[build] Extend rules/config.user to more Makefiles (#7344)

rules/config.user allows overriding default properties without
touching tracked files. This change makes sure all properties
can be set and not just the ones used in slave.mk.

Signed-off-by: Christian Svensson <blue@cmd.nu>

Makefile.work

@@ -100,6 +100,7 @@ SLAVE_DIR = sonic-slave-jessie
 endif
 
 include rules/config
+-include rules/config.user
 
 ifeq ($(ENABLE_DOCKER_BASE_PULL),)
 	override ENABLE_DOCKER_BASE_PULL = n