Updated optee_utils.c to fix ARM-software/tf-issues#603 related to the
tee-validate-header bug.

Minor updates to the header valid checking logic. It would never make
sense to have less than 1 image to load so this is now checked.

Changed OPTEE_MAX_IMAGE_NUM to OPTEE_MAX_NUM_IMAGES to clarify its
definition. OPTEE_MAX_IMAGE_NUM sounds like an ID assigned to the last
image to load. OPTEE_MAX_NUM_IMAGES sounds like the maximum number of
images to load.

Signed-off-by: Nathan Menhorn <nathan.menhorn@xilinx.com>