K3 Pico ITX EC update

The EC image is supplied separately. Firmware never updates it at boot.
Use the board's USB path as K3_USB; see INSTALL.txt for entering Fastboot.

1. Query

  fastboot -s "$K3_USB" getvar version-ec

Supported by coreboot RAM Fastboot (after recovery-bootrom.bin starts)
and EDK2 Fastboot (F9 or RAM recovery). Both also display the EC version.

2. Write

Use the vendor ec.bin for this board and check its vendor checksum first.
The inspected 0.0.31 package contains V00.21, 242688 bytes, SHA256:
  59d6be2700311685219c7a763324b0e8228de586d24966eb0a2dc7bc0d2f10e4

  fastboot -s "$K3_USB" flash ec ec.bin

Wait for verification to finish and OKAY. The command validates the image,
erases the first 256 KiB of EC flash, writes it and compares every byte.
Use the ec target; spi and firmware address the main board firmware.
Keep power connected. If writing or verification fails, retry before
restarting the EC. The update has no power-loss rollback.

3. Activate

  fastboot -s "$K3_USB" oem reboot-ec

Wait at least 15 seconds, then reset the board. In EDK2, fastboot reboot
can perform this reset; coreboot RAM Fastboot needs a manual board reset.
EC communication can be unavailable until the board restarts. Re-enter
Fastboot and query version-ec to confirm the new version is running.

Optional backup (EDK2 only)

  fastboot -s "$K3_USB" fetch ec ec-backup.bin

This is a raw 256 KiB backup, not a directly flashable ec.bin. The native
image length is 32 plus the little-endian 32-bit value at offset 12;
validate the container and retain the raw backup before extracting it.

See validation.json in the release package for the tested image hashes
and hardware scope. Interrupted-update recovery is not verified.