local.conf.in: Document shorter KVERSION

This is no longer needed to synchronize the kernel header versions.
It is currently only used in the package naming so use just the
major and minor kernel version numbers to avoid having to constantly
update it.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Merge pull request #2 from drewmoseley/kversion-update

local.conf.in: Document shorter KVERSION