mx7dsabresd: enable DFU support

mx7dsabresd: enable DFU support

On the target board:

  => dfu 0 mmc 0

On the host PC:

  $ dfu-util -l

    Found DFU: [0525:a4a5] devnum=0, cfg=1, intf=0, alt=0, name="image"
    Found DFU: [0525:a4a5] devnum=0, cfg=1, intf=0, alt=1, name="u-boot"
    Found DFU: [0525:a4a5] devnum=0, cfg=1, intf=0, alt=2, name="bootimg"
    Found DFU: [0525:a4a5] devnum=0, cfg=1, intf=0, alt=3, name="rootfs"

  Update with a full SD image

  $ dfu-util -a image -D core-image-minimal-imx7dsabresd.sdcard

  Update u-boot only

  $ dfu-util -a u-boot -D u-boot.imx

Signed-off-by: Tzu-Jung Lee <roylee17@currantlabs.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>