boards: bbc_microbit: add nrfjprog flash support

Segger offers J-Link firmware for the debug MCU on the BBC micro:bit.
If this firmware is installed, it allows programming via nrfjprog and
JLink. Add support for this in the board.cmake.

Leave the default at pyocd, to support the factory-programmed
firmware.

With this patch, users who have installed the JLink firmware can flash
this board with nrfjprog using:

$ west flash -r nrfjprog

And can flash or debug with jlink using:

$ west flash -r jlink
$ west debug -r jlink

As usual with runner overrides, you can omit the '-r {nrfjprog,jlink}'
if BOARD_FLASH_RUNNER as a CMake variable is set to nrfjprog or
BOARD_DEBUG_RUNNER is set to jlink.

For more details on this Segger firmware, see:

https://www.segger.com/products/debug-probes/j-link/models/other-j-links/bbc-microbit-j-link-upgrade/



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>