cassini-distro: Use PACKAGECONFIG to select the Parsec config file

The logic to configure the default Parsec provider is currently
duplicated in both the distro config and the bbappend.

Remove the duplication by reusing the PACKAGECONFIG to set
PARSEC_CONFIG.

This also provides additional flexibility in case a Cassini distro user
wishes to use a different provider, without affecting the default.

Changelog: other
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>