include: make dtb argument optional in MkFIT

As mkits.sh makes the -d argument optional, we'll make this same
argument optional in the MkFIT function as well.

With this change, MkFIT can be used both to generate FIT images with DT,
and without DT.

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>

SVN-Revision: 45660