meta-google: os-release: add GBMC_TARGET_MACHINE

Tested:
Check
`tmp/work/all-openbmc-linux/os-release/1.0-r0/package/etc/os-release`
and the `GBMC_TARGET_MACHINE` has the value as expected.

Change-Id: I8b2d2b2aefcd3310f36404f97af0d03f840f64ad
Signed-off-by: Tom Tung <shes050117@gmail.com>
diff --git a/meta-google/recipes-core/os-release/os-release.bbappend b/meta-google/recipes-core/os-release/os-release.bbappend
index 635f7c5..0d4904c 100644
--- a/meta-google/recipes-core/os-release/os-release.bbappend
+++ b/meta-google/recipes-core/os-release/os-release.bbappend
@@ -8,4 +8,6 @@
 }
 
 IMAGE_TYPE = "${GBMC_CONFIG}"
-OS_RELEASE_FIELDS:append:gbmc = " BUILD_MEMO IMAGE_TYPE"
+GBMC_TARGET_MACHINE ?= "${MACHINE}"
+
+OS_RELEASE_FIELDS:append:gbmc = " BUILD_MEMO IMAGE_TYPE GBMC_TARGET_MACHINE"