scripts: pyocd.sh: Add support for passing board_id to pyocd commands

If we have more than one board of a given type we need to be able to
specify the board_id to select which specific board that the pyocd
command should target.  Introduce an environment variable PYOCD_BOARD_ID
to we can set that will get passed to the pyocd command that needs it.

Here's an example:

$ make -C samples/hello_world/ BOARD=frdm_k64f flash PYOCD_BOARD_ID=1234

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