More complete control over KERNELVERSION variable

Allow using revision information (from svn or git) even if the version
number is changed on the command line (eg. make KERNELVERSION='11.03$(REV)')
or dropping it entirely if having that information in the coreboot binary is
not desired.

Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Acked-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6449 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1