More complete control over KERNELVERSION variable

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