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