boards: hexiwear_{k64, kw40z}: Rework programming and debugging doc

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

Recommends the OpenSDA J-Link debug probe because the k64 and kw40z
share the same OpenSDA microcontroller, and the kw40z requires Segger
RTT for a console. It is possible to use daplink firmware for the k64,
however it is not recommended because it requires switching the firmware
back to jlink to access the kw40z.

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