st/mesa: Fix paths used in Android builds

st/mesa: Fix paths used in Android builds

With current makefiles the build fails because source and build paths
are generated incorrectly. With Android build system the top_srcdir and
top_builddir variables are undefined and all paths are relative to where
Android.mk is located. This ends up with path likes
external/mesa/src/mesa/src/mesa/ for both source and build paths, which
are obviously wrong.

This patch fixes this by overriding resulting SRCDIR and BUILDDIR
variables with empty string, so that paths end up being relative to
Android.mk file again. Appending correct build path to generated files
is already done in Android.gen.mk.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>