This removes duplication for the jlink case (keeping only any
non-default values in the board makefiles), and opens up STK3700's
OpenOCD programmability for generic boards.
This is still currently a hack to hardcode it as the value can be deduced
from the `BOARD_MODULE` daughter board name.
But it requires more cleanup and could come in a separate step.
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.