Demonstrate DFU usage for DHCOR recovery

This patch configures U-Boot SPL for DHCOR SoM to permit DFU upload of SPL and subsequent u-boot.itb for recovery or commissioning purposes. It is not possible to use this SPL build for production due to its large size.

Compile U-Boot for the DHCOR Avenger96 target: $ make stm32mp15_dhcor_basic_defconfig $ make -j`nproc`

- Install dfu-util on the host PC (in debian this is package 'dfu-util')

- Power off the Avenger96 board.

- Connect both USB-serial console and USB-OTG microB ports to host PC.

- Switch Avenger96 to USB boot mode -- BOOT0..2 switches all set to 0.

- Power on the Avenger96 board.

- Verify using '$ dmesg' that a new device has been detected as follows:

   New USB device found, idVendor=0483, idProduct=df11, bcdDevice= 2.00
   New USB device strings: Mfr=1, Product=2, SerialNumber=3
   Product: DFU in HS Mode @Device ID /0x500, @Revision ID /0x0000
   Manufacturer: STMicroelectronics

- Upload U-Boot SPL:

$ dfu-util -a 1 -D u-boot-spl.stm32

- Upload U-Boot proper:

$ dfu-util -a 0 -D u-boot.itb

At this point, SPL will wait for user to press "Ctrl-C" on serial console. When ready to interact with U-Boot, press Ctrl-C to start the bootloader.

Note that this is adapted from ST article, with removed requirement for custom DFU implementation and additional bootloader components, which are not required on this platform: https://wiki.st.com/stm32mpu/wiki/How_to_load_U-Boot_with_dfu-util

Signed-off-by: Marek Vasut's avatarMarek Vasut <marex@denx.de>