doc: Update NXP board docs for pyOCD and Segger J-Link

Now that NXP boards support 'make flash' and 'make debug' via pyOCD and
Segger J-Link, update all the NXP board docs accordingly. Adds a new
generalized OpenSDA document so certain details don't have to be
repeated for every OpenSDA board.

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