CI: enable debug-tweaks IMAGE_FEATURE

This makes the images suitable for development and test,
debug-tweaks includes settings like empty root password.

All CI image builds inheriting from base.yml will
have this include this setting.

Signed-off-by: Anastasios Kavoukis <anastasios.kavoukis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>