qoriq: use FIT uImage for Firebox M300 kernel

qoriq: use FIT uImage for Firebox M300 kernel

This requires U-Boot environment changes:

  setenv OpenWrt_kernel watchguard_firebox-m300-fit-uImage.itb
  setenv loadaddr 0x20000000
  setenv wgBootSysA 'setenv bootargs root=/dev/mmcblk0p2 rw rootdelay=2 console=$consoledev,$baudrate fsl_dpaa_fman.fsl_fm_max_frm=1530; mmc dev 0; ext2load mmc 0:1 $loadaddr $OpenWrt_kernel; bootm $loadaddr'

Trying to sysupgrade an image containing this change on an M300 already
running OpenWrt will fail with the following error:

  Tue Jun 14 12:06:21 EEST 2022 upgrade: The device is supported, but the config is incompatible to the new image (1.0->1.1). Please upgrade without keeping config (sysupgrade -n).
  Tue Jun 14 12:06:21 EEST 2022 upgrade: Kernel switched to FIT uImage. Update U-Boot environment.
  Tue Jun 14 12:06:21 EEST 2022 upgrade: Reading partition table from bootdisk...
  Tue Jun 14 12:06:21 EEST 2022 upgrade: Extract boot sector from the image
  Tue Jun 14 12:06:21 EEST 2022 upgrade: Reading partition table from image...
  Image check failed.

This is to prevent rendering your device unbootable. Make the U-Boot
environment changes as instruced above, and then flash the image using
sysupgrade -F. The config can be kept, there is no need to use -n.

After the new image booted successfully, you can increase the compat_version:

  uci set system.@system[0].compat_version='1.1'
  uci commit

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
(cherry picked from commit c4b499bc03ab0e2eea643c46d1d781ab64e78931)