image.h: make image_sign_info.fit point to const

image.h: make image_sign_info.fit point to const

The data blob apparently does not need to be modified through the fit
field of the image_sign_info struct so make it point to const to avoid
the need to cast away constness in functions that assign a pointer to
const data to the field.

fit_image_setup_verify already had to cast away constness as it assigned
a const void * argument to the field. The cast can now be removed.

Signed-off-by: Hannu Lounento <hannu.lounento@vaisala.com>
Reviewed-by: Simon Glass <sjg@chromium.org>