Only attach reset button interrupt for specific variants

This may be too many variants: the removed comment suggests only the
xplain board needs this functionality. But for now I'm just keeping the
functionality identical for these boards, since I can't test them.
Add board/variant for Amaret LED Wearable V1

Largely copied from the Akafuino stuff, but we don't want any of that
reset button stuff. The digital pins setup is also wrong/unnecessary; we
can get to that later.