Mark envsetup.sh vars as deprecated in makefiles

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

README.md

@@ -0,0 +1,23 @@
+# Android Make Build System
+
+This is the Makefile-based portion of the Android Build System.
+
+For documentation on how to run a build, see [Usage.txt](Usage.txt)
+
+For a list of behavioral changes useful for Android.mk writers see
+[Changes.md](Changes.md)
+
+For an outdated reference on Android.mk files, see
+[build-system.html](/core/build-system.html). Our Android.mk files look similar,
+but are entirely different from the Android.mk files used by the NDK build
+system. When searching for documentation elsewhere, ensure that it is for the
+platform build system -- most are not.
+
+This Makefile-based system is in the process of being replaced with [Soong], a
+new build system written in Go. During the transition, all of these makefiles
+are read by [Kati], and generate a ninja file instead of being executed
+directly. That's combined with a ninja file read by Soong so that the build
+graph of the two systems can be combined and run as one.
+
+[Kati]: https://github.com/google/kati
+[Soong]: https://android.googlesource.com/platform/build/soong/+/master