linux: remove default for custom kernel version

There already is a default kernel version, so it makes little sense to have
a default custom kernel version as well.  This default breaks the 'make
savedefconfig' if we want to force a specific version.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>