README.md: fix --tunOut /gwName usage
Add Android.mk to compile with the Android NDK

Also, some more fixes that were preventing it from building with the
cross-compiler.
Re-add -Wall to Android.mk
Add a very basic .gitignore
Moved common stuff into a separate makefile
Minor changes to improve readability
Also accept .git as a file in case we are a submodule

Also, the usage of REVISION_VERSION is a bit strange. It's much easier to
avoid redefining GIT_REV, so that we can use "make GIT_REV=foo" or
"GIT_REV=foo make".
Use Common.mk from Android.mk