meta-google: Add GBMC_CONFIG variable

GBMC_CONFIG variable controls dev/prod configuration for the builds.
Possible values: dev, prod

This variable is defaulted to "dev" through weak assignment.
Can be explicitly set through local.conf file or in build command.
To set it in build command, additional step of including this variable
to the "BB_ENV_EXTRAWHITE" environment variable is needed.

https://docs.yoctoproject.org/bitbake/bitbake-user-manual/bitbake-user-manual-metadata.html#passing-information-into-the-build-task-environment



Signed-off-by: Vivekanand Veeracholan <vveerach@google.com>
Change-Id: I46c34d47c958a96ce00e2c89a2bc771a483a83ab