linux: explicit latest kernel version

When setting BR2_LINUX_KERNEL_LATEST_VERSION, it is hard for the user to
know that this version is subject to change in the future.

Explicit this in the Kconfig entry text.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>