Driver core: exclude kobject_uevent.c for !CONFIG_HOTPLUG

Move uevent specific logic from the core into kobject_uevent.c, which
does no longer require to link the unused string array if hotplug
is not compiled in.

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