Summary: As Aaron suggested, there are quite some problems with our Makefile and scripts. So in this diff I did some cleanup for them and revise some part of the scripts/makefile to help people better understand some mysterious parts.

Test Plan:
Ran make in several modes;
Ran the updated scripts.

Reviewers: dhruba, emayanke, akushner

Differential Revision: https://reviews.facebook.net/D12285