arm: add a release conf for distribution files

We do not currently generate armv7 distribution sets, because we don't
build any installer images. However, having such sets available can be
useful for quickly installing a base system, particularly in the case
of creating an armv7 poudriere jail on arm64.

Add a configuration file for the generation of these distribution sets.

Reviewed by: manu, imp, gjb
MFC after: 3 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D29923