The kwbimage detection calculates a checksum over the first 31 bytes of
the image. This is correct for the v0 image format, but for the v1 image
format the checksum in the image also covers the extenstion headers.
These might not be completely present in the initial buffer provided to
file_detect_type(), so just drop the checksum calculation for v1 images.

Fixes: bf8b6d46db ("kwbimage_v0: add support to detect and boot a mvebu v0 image")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>