targets/audio: If commit id is not found, fall back on master

Can happen with developer builds using prebuilt packages, see
crbug.com/417820. In this case, we assume that the developer
will update the machine frequently and using master is
appropriate.
audio: If master is picked, audio might not work
Merge pull request #1076 from drinkcat/cras-version-master-fallback

targets/audio: If commit id is not found, fall back on master