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 157a5e16.)

Bug: 7186768,7169854
Change-Id: I0be5e2721efa66a80f112acf8f95e177ebc2f65b