tbbr/dualroot: Add fw_config image in chain of trust

tbbr/dualroot: Add fw_config image in chain of trust

fw_config image is authenticated using secure boot framework by
adding it into the single root and dual root chain of trust.

The COT for fw_config image looks as below:

+------------------+       +-------------------+
| ROTPK/ROTPK Hash |------>| Trusted Boot fw   |
+------------------+       | Certificate       |
                           | (Auth Image)      |
                          /+-------------------+
                         /                   |
                        /                    |
                       /                     |
                      /                      |
                     L                       v
+------------------+       +-------------------+
| fw_config hash   |------>| fw_config         |
|                  |       | (Data Image)      |
+------------------+       +-------------------+

Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I08fc8ee95c29a95bb140c807dd06e772474c7367