buildroot

cmake: expose the host variant in menuconfig

With the addition of the sdk support it may be interesting to build
host-cmake even though no packages need it, so expose it in menuconfig.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>