drivers: fastboot: zImage flashing is not supported for v3, v4

With vendor boot image introduced in version 3 and 4 of boot
image header, boot information is located in both boot image
and vendor boot image.

Flashing zImage is not supported for version 3 and 4 since this
requires updating vendor boot image and/or generating a new image.

Print an error message when the boot image header version is
greater than 2.

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>