mosys: Add a fallback option when no valid sku id is found

To support Dru where RO firmware doesn't export sku-id to FDT,
we need to have a default sku id to fall back to. Otherwise,
in reocvery mode, firmware update for Dru will fail after
CL:*693332 lands.

BUG=b:115771305
TEST=boot Dru with a trybot image and flash the old BIOS (10388.26.0)
which doesn't export sku id, confirm firmware updater can
still detect model == dru and update firmware successfully

Change-Id: Idc6596c83f932b238c33773137fb19126dce2377
Reviewed-on: https://chromium-review.googlesource.com/1284810
Commit-Ready: Philip Chen <philipchen@chromium.org>
Tested-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
4 files changed