boards: mimxrt10{20,50}_evk: Rework programming and debugging doc

Reworks the programming and debugging section in the mimxrt1020_evk and
mimxrt1050_evk board documents to leverage the new debugging guide
covering debug probes and host tools.

These boards support both the OpenSDA J-Link onboard debug probe and the
J-Link external debug probe.

Corrects an error linking to the wrong OpenSDA J-Link firmware.

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

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