kernel.bbclass: use better number for KERNEL_PRIORITY

* there is no upgrade from 2.6.X to 3.X.Y
  last part of PV is used as kernel priority for u-a, but X is usually higher then Y in 3.x.x
  so use all 3 parts in one bigger number
* and make it weak assignment if this scheme doesn't work for some
  recipe

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>