EC2: Add extra SSM Parameter names for releases

Record releases in the SSM Parameter Store not just as e.g.

/aws/service/freebsd/arm64/small/ufs/15.0/RELEASE

but also as .../RELEASE/latest, e.g.

/aws/service/freebsd/arm64/small/ufs/15.0/RELEASE/latest

The latter parameters will be updated when new AMIs are built with
security and errata updates appplied, and have been backfilled for
15.0.

MFC after: 3 days
MFC to: stable/15 (14.x will not get security-update AMIs)
Sponsored by: Amazon