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
diff --git a/meta-google/conf/distro/gbmc.conf b/meta-google/conf/distro/gbmc.conf
index d365909..50fdc59 100644
--- a/meta-google/conf/distro/gbmc.conf
+++ b/meta-google/conf/distro/gbmc.conf
@@ -7,6 +7,10 @@
 DISTRO_VERSION = "gbmc-release-${GBMC_VERSION}"
 DISTROOVERRIDES .= ":gbmc"
 
+# Possible values: "dev", "prod"
+GBMC_CONFIG ?= "dev"
+OVERRIDES .= ":${GBMC_CONFIG}:"
+
 DISTRO_FEATURES:remove = "ldap"
 DISTRO_FEATURES:remove = "slp"