Starting with FreeBSD 11 we will use dvd1 disk instead of disc1, since we
need kernel-dbg distribution. For 11.0-BETA1 and 11.0-BETA2 this was done
as override, but we should use it as default.

In case if 10.4-RELEASE will use disc1, we will create override for 10.4
build only.

Discussed with: delphij, gjb