meta-google: os-release: Add dev/prod image type

Add dev/prod image type to /etc/os-release.

Tested:
Built with GBMC_CONFIG=dev bitbake obmc-phosphor-image

```
cat /etc/os-release
ID=gbmc
NAME="gBMC (OpenBMC + Google customizations)"
VERSION="gbmc-release"
VERSION_ID=gbmc-release-0.5.0.0
PRETTY_NAME="gBMC (OpenBMC + Google customizations) gbmc-release"
BUILD_ID="2.11.0-dev"
IMAGE_TYPE="dev"
```

Change-Id: I355e6069b08107717fb406b6b603ce4ce5264ff3
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/meta-google/recipes-core/os-release/os-release.bbappend b/meta-google/recipes-core/os-release/os-release.bbappend
index 8c61bbc..f6b7cc1 100644
--- a/meta-google/recipes-core/os-release/os-release.bbappend
+++ b/meta-google/recipes-core/os-release/os-release.bbappend
@@ -7,4 +7,5 @@
         d.setVar("BUILD_MEMO", memo)
 }
 
-OS_RELEASE_FIELDS:append = " BUILD_MEMO"
+IMAGE_TYPE = "${GBMC_CONFIG}"
+OS_RELEASE_FIELDS:append = " BUILD_MEMO IMAGE_TYPE"