buildroot

s6-rc: expose the host variant in menuconfig

Some users may want to build the s6-rc service database offline using a
post-build script. So make the host variant visible in menuconfig.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>