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
ac099daf67