fit: Fix verification of images with external data

The "-E" option to mkimage generates a FIT with external data using the
data-size and data-offset properties which must both be ignored when
verifying a signature.

Add "data-offset" to the list of excluded properties for signature
verification; since the line is now too long, re-format the list to
one-per-line and make it static since the data is constant.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>