make-memstick.sh: use UFSv2

There's not much practical difference as far as install media is
concerned but newfs creates UFSv2 by default and it is sensible to use
the contemporary UFS version.

I also intend to change makefs to create UFSv2 by default (to match
newfs) so we'll want make-memstick.sh to be explicit, rather than
relying on the host tool's default.

Reviewed by: andrew, gjb, jhibbits
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12231