[makefile] define a do-nothing target for config.user (sonic-net#7483)

Why I did it
After PR sonic-net#7344, 'make init' and/or 'make reset' will also build sonic slave dockers.

'-include rules/config.user' is supposed to be fine when the file is missing. However, when the file is missing, it generates a delayed error which later causes make init and make reset trying to build the sonic slave dockers.

How I did it
Define a do-nothing target for config.user to catch config.user build therefore preventing other builds to be triggered unexpectedly.

How to verify it
did make init and it is now only doing submodule init.

Makefile.work

@@ -99,6 +99,11 @@ else
 SLAVE_DIR = sonic-slave-jessie
 endif
 
+# Define a do-nothing target for rules/config.user so that when
+# the file is missing, make won't try to rebuld everything.
+rules/config.user:
+	@echo -n ""
+
 include rules/config
 -include rules/config.user