Cull armv6/armv7 boards
Boards we currently provide release images for (11.x):
GUMSTIX is a company, not a model, and I'm not sure which specific board it is, but I strongly suspect we can drop everything pre-armv6 at least.
GUMSTIX was removed in r336776
I've added a GENERICSD image in r336848
gjb don't want to remove other board, see https://reviews.freebsd.org/D16237