Driver core: add CONFIG_UEVENT_HELPER_PATH

The kernel creates a process for every event that is send, even when
there is no binary it could execute.  We are needlessly creating around
200-300 failing processes during early bootup, until we have the chance
to disable it from userspace.

This change allows us to disable /sbin/hotplug entirely, if you want to,
by setting UEVENT_HELPER_PATH="" in the kernel config.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>