Load only the current product config makefile.

To reduce the start time.
With the change and previous change of doing clean step in only given paths,
start time of mm/mmm is reduced from ~5s to about 1s;
lunch time is reduced from ~40s to 3.5s.

(cherry-pick of 157a5e1695593f935c3223430f4530e21f990378.)

Bug: 7186768,7169854
Change-Id: I0be5e2721efa66a80f112acf8f95e177ebc2f65b
diff --git a/core/config.mk b/core/config.mk
index 03e4245..a894444 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -14,6 +14,11 @@
 SHELL := /bin/bash
 endif
 
+# Utility variables.
+empty :=
+space := $(empty) $(empty)
+comma := ,
+
 # Tell python not to spam the source tree with .pyc files.  This
 # only has an effect on python 2.6 and above.
 export PYTHONDONTWRITEBYTECODE := 1