Reland "Remove Gru platform"

This reverts commit cdd7f8cf5c6da4a2cd53c9e77810468b02f450b5.

Reason for revert: go/cros-unmonitored-builds has completed, cleaning up.

Original change's description:
> Revert "Remove Gru platform"
>
> This reverts commit d6c45f445712bbf36b58f5b1e9d1c588e461dd56.
>
> We need 'gru' so that we can build 'scarlet' as part of
> http://go/cros-unmonitored-builds
>
> BUG=b:321067912
> TEST=build and deploy a scarlet build to a dru
>
> Change-Id: I3fcc5ee622425fa8368828e342eda3e74bb023ca
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/5318541
> Reviewed-by: Jack Rosenthal <[email protected]>
> Tested-by: Ross Zwisler <[email protected]>
> Commit-Queue: Ross Zwisler <[email protected]>
> Auto-Submit: Ross Zwisler <[email protected]>

BUG=b:321067912 b:321077155 
TEST=none

Change-Id: Ie43e71c313a205dd0ac2f84f5422f80098950f59
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/6065239
Reviewed-by: Yu-Ping Wu <[email protected]>
Commit-Queue: Yu-Ping Wu <[email protected]>
Auto-Submit: Ross Zwisler <[email protected]>
Tested-by: Ross Zwisler <[email protected]>
4 files changed