board: gateworks: venice: dynamically update the update_firmware script

board: gateworks: venice: dynamically update the update_firmware script

The update_firmware script is intended to update the boot firmware but
the details including the offset and hardware partition are dependent
on the boot device.

Specifically:
- IMX8MM/IMX8MP (BOOTROM v2) the offset is 32KiB for SD and eMMC user
  hardware partition and 0KiB for eMMC boot partitions.
- IMX8MM the offset is 33KiB for SD and eMMC regardless of hardware
  partition.

Dynamically set soc, dev, bootpart, and bootblk env vars at runtime
and use these in the update_firmware script. Remove the splblk env var
from config files as its no longer needed.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>