Boot an LE image. Connect a 16GB (or larger) USB stick, formatted EXT4. Run "dd if=/dev/device of=/var/mount/USB/backup.img bs=1m" to clone the contents of emmc to a file on the USB drive. The /dev/device will be something like /dev/mmcblk0 .. whatever the internal emmc is, and where USB is the disk label for the USB drive. You'll have to figure out what these are. Once the backup is complete (it will take time) .. "dd if=/dev/zero of=/dev/device bs=1m" will erase the emmc. Once emmc is erased (and vendor u-boot is gone) the WP2 will find mainline u-boot on the SD card and use it.
in putty windows line dd if=/dev/device of=/var/mount/USB/backup.img bs=1m
dd: invalid number '1m'