Deprecating old SDK entry mechanisms

[Benjamin Gilbert]

Hi all,

As of mantle 0.9.0, “cork enter” now defaults to the new mechanism for entering the SDK (previously accessible via "cork enter --experimental").  Two older mechanisms have been deprecated and will soon be removed:

- The old “cork enter”, now accessible via “cork enter --legacy”
- “chromite/bin/cros_sdk”

Please switch to using “cork enter” on mantle 0.9.0 or greater.  If you encounter any problems with this mechanism, please let us know here or file a bug.

Thanks,
--Benjamin Gilbert