buildroot

linux: use BR2_LINUX_KERNEL_LATEST_VERSION as option name

Use a option name that doesn't change for every new 3.x version.
It's easier to maintain and consistent with barebox.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>