Use makefs -t msdos in make_esp_file

With this last piece in place, make -C /usr/src/release release.iso is
finally able to run in a jail. This was not possible before because
msdosfs cannot be mounted inside a jail.

Submitted by: ryan@ixsystems.com
Reviewed by: emaste@, imp@, gjb@
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D21385