meta-phosphor: generate supported machine list

Add a script to generate a list of supported machines and hook it
into the `run-repotest` to ensure that it is ran as necessary when
new machines are added or removed.

Fixes openbmc/openbmc#3926.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If33aa08451bb8a789459bf49d8d7ec9044973304