Consolidate conversions of is-board-platform(2)

mk2rbc implements conversions for both
is-board-platform(-in-list) and
is-board-platform(-in-list)2. The conversions
are not the same despite the functions being the
same. Make them convert to the same thing.

Bug: 201477826
Test: go test
Change-Id: I688a2f6114e1688c713e42f0f9229cbc5ee6a917