board: samsung: e850-96: Enter DFU automatically on USB boot

Doing USB boot on E850-96 is most useful in two cases:

  1. For unbricking the board
  2. During the bootloader development

In both cases a U-Boot binary is being re-flashed to eMMC. The most
convenient way to update U-Boot in eMMC is by using DFU. Implement
entering DFU flashing mode automatically when U-Boot is executed on USB
boot. That makes it easier for users to re-flash U-Boot without even
having serial console running, e.g.:

    $ ./smdk-usbdl
    $ dfu-util -D u-boot.bin -a bootloader

See [1,2] for details.

Entering DFU mode is implemented by setting corresponding environment
variables:

    bootcmd="dfu 0 mmc 0"
    bootdelay=0

Do not save the U-Boot environment though, to avoid falling through to
DFU mode on a regular eMMC boot.

[1] doc/board/samsung/e850-96.rst
[2] https://gitlab.com/LinaroLtd/e850-96/tools/dltool/-/tree/uboot

Signed-off-by: Sam Protsenko <[email protected]>
Signed-off-by: Minkyu Kang <[email protected]>

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