config: move image field to hafnium-base

Follow up to:
a502ce34 feature: add an optional 'image' field to config files.
0ca0537d config: ffa-hafnium-optee: Use the optional 'image' field

Move the "image:" field to hafnium-base config and remove it from
cca-4world and ffa-hafnium-optee configs.

This will benefit configurations consuming hafnium-base config including
above ones and ffa-tftf, or future configuration consuming this config.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>