boards: mimxrt10{60,64}_evk: Rework programming and debugging doc

Reworks the programming and debugging section in the mimxrt1060_evk and
mimxrt1064_evk board documents to leverage the new debugging guide
covering debug probes and host tools.

Neither of these boards have OpenSDA J-Link board-specific firmware,
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>