config: ffa-hafnium-optee: Use the optional 'image' field

ffa-hafnium-optee configuration requires shrinkwraptool/base-full docker
image similar to cca-4world configuration. This requirement is currently
exposed on the command line by expecting as the user to specify the right
image when invoking shrinkwrap. This seems unnecessary and error prone.

Commit a502ce34 ("feature: add an optional 'image' field to config files.")
added option to override the default docker image file used by specifying
the optional 'image' field in the config file.

Lets us relax the command line expectation and set 'image' field to
shrinkwraptool/base-full for the ffa-hafnium-optee configuration.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>