nw: emmc on SDHC4, not SD on SDHC3 for rev2 boards

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nw: fix eMMC startup

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Add nw-usbwrite_defconfig for use in programmin eMMC

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
nw-usbwrite: prune defconfig

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