backports: re-introduce make uninstall

This approach only uninstalls modules you have built
previously on your current build directory for backports.
If this requires adjustments then we'll have to just have
to use our own new backports KMODDIR.

Signed-off-by: Felix Bitterli <ic.felix@gmail.com>
[mcgrof: extended the commit log a bit]
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>