Update debian/rules to upstream Makefile changes

Pass build options to make instead of through Makefile.local. Upstream
removed support for Makefile.local in their buildsystem update, so we
just pass these options to the make command directly.

Also explicitely build the "doc" target in the build step, since
building documentation no longer happens automatically.

Move the removal of prebuilt graphics files from configure to
build, it makes more sense there (and there are no other things to do in
the configure step anymore).

Finally, actually install the changelog, license and documentation files
into /usr/share/games/openttd/data/opengfx, to allow viewing them from
inside OpenGFX. Prevent installing the changelog and readme twice by
providing symlinks in /usr/share/doc/openttd-opengfx, and link the
license.txt to /usr/share/common-licenses.