package/docker-engine: needs some kernel options

according to https://wiki.gentoo.org/wiki/Docker#Kernel

 /
docker-engine/contrib/check-config.sh.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Peter: synchronize with contrib/check-config.sh, only enforce DM options if
 device mapper driver is enabled]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>