cmake: dts: Fix usage of BOARD vs BOARD_FAMILY

We assumed that ${BOARD} and ${BOARD_FAMILY} are always the same and
thus interchangeable.  That isn't always the case so use ${BOARD} proper
for when we mean the exact BOARD name we are building for vs the
BOARD_FAMILY.

We also add .dts files for board variants within a board family, such
that we have a .dts matching each board name.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>