release/tools: Add support for building armv7 vm

Currently there is no support for generating armv7 vm images in the
release artifacts. In fact in terms of release artifacts and
architecture there is no good reason to have a vm release artifact for
armv7 as those are mostly used in SOCs or embedded boards. However
considering that developers actually do need an easy way to test armv7
with a vm running this is really important. As part of pre-commit ci for
developers this can be really helpful for the end developers.

Approved by: cperciva, imp, re
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43952