scripts: Add flash and debug support using pyOCD on NXP boards

Adds a new debug support script using pyOCD and configures most NXP
boards so they can use it. frdm_kw41z is the one exception because pyOCD
doesn't yet support kw41z. Tested with pyOCD v0.8.0 and the latest
DAPLink firmware for each board.

Introduces two new environment variables, PYOCD_FLASHTOOL and
PYOCD_GDBSERVER, that allow you to set custom paths to the pyOCD tools.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>