cassini-config: Align YML files for base configs

For clarity, the YML files for the three cassini base configs should
all have the same structure.

Align cassini-dev.yml/dev.yml and cassini-sdk.yml/sdk.yml with
cassini.yml such that:
- cassini-<foo>.yml includes cassini-base.yml and sets the target
- cassini-<foo>.yml includes <foo>.yml
- <foo>.yml adds the distro feature cassini-<foo>

Signed-off-by: Adam Johnston <adam.johnston@arm.com>