kernel.bbclass: restore kernel-abiversion file

This fixes external module recipes that need $KERNEL_VERSION. It got removed by

	commit a9d41062e24a6b99661b3a5256f369b557433607
	Author: Darren Hart <dvhart@linux.intel.com>
	Date:   Tue Mar 8 17:09:10 2011 -0800

		kernel/bbclass: rework kernel and module classes to allow for building out-of-tree modules

seemingly as an oversight.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>