Why I did it
Some Arista products do not have an SSD but use an eMMC instead.
The SsdUtil plugin is therefore extended to support both.
How I did it
Implemented ssd_util.py platform plugin loaded by ssdutil.
This plugin fallback to the default SONiC implementation if the arista one can't be found.
How to verify it
Run show platform ssdhealth on a product with an eMMC
