Adjust main Makefile to allow automating Android builds.

Assuming you have the Android SDK and NDK in PATH, use:
make ANDROID=y TILES=y android

It's far from flawless right now, but it's a decent start and it gets
the data files into a place where they will be usable from the Android
build once we write the code to hook it up.

TODO:
  - export the cflags from here for use within the Android project files
    so that they're not hard-coded; finding a way to do that with the
    source files would be good too (but seeing as we already have several
    hard-coded lists of source files that one's not as critical);
  - allow easy toggling between debug and release Android builds.
Remove a couple of files from the old Android build.

People might get confused as to why these are here, since they have
nothing to do with the SDL2 Android build.