[build]: Add a user defined rules/config file not tracked by git (#5325)

This change introduce the rules/config.user file.
It gets loaded after rules/config as a mean to override default values.
If the configuration file does not exists this step is ignored.
Since this path is tracked by .gitignore, it provides a mean to change
the configuration of sonic build without dirtying the git tree.

Signed-off-by: Samuel Angebault <[email protected]>

Author: Staphylo

.gitignore

@@ -5,6 +5,7 @@
 .arch
 .platform
 .screen
+rules/config.user
 
 # Build artifacts
 fsroot/

slave.mk

@@ -94,6 +94,7 @@ list :
 ###############################################################################
 
 include $(RULES_PATH)/config
+-include $(RULES_PATH)/config.user
 
 ifeq ($(SONIC_ENABLE_PFCWD_ON_START),y)
 ENABLE_PFCWD_ON_START = y