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.

Bug: 7186768,7169854
Change-Id: Ifd72ba1c6362a5e1125a23c505fd47947ab56675
diff --git a/core/config.mk b/core/config.mk
index 3924689..ef92268 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