kbuild: make it possible to specify the module output dir

Make modinst_dir user-defined on the command line.

This allows to do things like:
make MODLIB=output/ modinst_dir=. modules_install

to ensure all the .ko are in the output/ directory.

Change-Id: I2bc007eea27ee744d35289e26e4a8ac43ba04151
Signed-off-by: Rom Lemarchand <romlem@android.com>
(cherry picked from commit ae7f28e5aa168a008c153ccff7cc437066d016c3)