utils/serializser: Re-fix support for YAML anchors
utils/serializser: Re-fix support for YAML anchors

Include missing `flatten_mapping` call in our implementation of
`construct_mapping`. This is performed by a subclass in the default
implementation which was missing in our previous fix.

ARM-software/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,setrofim/workload-automation