kernel.bbclass: pass KERNEL_VERSION to depmod calls in postinst

* without this, kernel upgrades where KERNEL_VERSION is changed
  e.g. 3.4.2 -> 3.4.3 generate .dep for running 3.4.2 and after reboot user ends
  up without any module loaded to make it worse after reboot nothing is upgraded
  to trigger another kernel(-module) postinst to generate .dep for now running 3.4.3

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>