tools: imagetool: Skip autodetection of gpimage type

gpimage type requires only that two first 32-bit words of data file are
non-zero. So basically every random data file can be guessed and verified
as gpimage. So completely skip gpimage type from image autodetection code
to prevent lot of false positive results. Data file with gpimage type can
be still verified and parsed by explicitly specifying -T gpimage.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>