boards: usb_kw24d512: Rework programming and debugging doc

Reworks the programming and debugging section in the usb_kw24d512 board
document to leverage the new debugging guide covering debug probes and
host tools.

This board does not have an OpenSDA microcontroller, therefore the only
debug probe currently supported is the external J-Link.

Updates the flashing section to reflect that the ``flash`` build system
target is now supported.

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