kbuild: Clean up and speed up the localversion logic

Now that we run scripts/setlocalversion during every build, it makes
sense to move all the localversion logic there. This cleans up the
toplevel Makefile and also makes sure that the script is called only
once in 'make prepare' (previously, it would be called every time due to
a variable expansion in an ifneq statement). No user-visible change is
intended, unless one runs the setlocalversion script directly.

Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>