Add CONFIG_CMD_MEMTEST to our configurations

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nitrogen6x 1066mhz_4x128m: use CAS latency, tMRD to 7 clocks

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nitrogen6x: add board and cpu environment variables for scripting devicetree loads

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>