For the envsetup.sh variables that should not be used in makefiles
(since they're not explicitly set up, and won't be available on the
build servers), mark them as deprecated.
Rework our documentation to have a landing page, and create a "Changes"
section where we can record changes like these. At some point I may go
and backfill some recent work.
Test: build/soong/build_test.bash
Change-Id: I54b9294ddf270245afdb58d17150db8098584e8a
Dan Willemsen