os-release: add EXTENDED_VERSION to /etc/os-release

This adds a new OS identification parameter (EXTENDED_VERSION) to
the /etc/os-release file in the generated OpenBMC image to indicate
the extended version of the OpenBMC image.

Note the extended version is typically set in the
openbmc/meta-*/meta-MACHINE/conf/local.conf.sample file
(This is where TEMPLATECONF points to.) by "EXTENDED_VERSION"
variable.

Tested: On the build system, `cat $IMAGE_ROOTFS/etc/os-release`
        shows the correct value.

Signed-off-by: Chanh Nguyen <chanh@amperemail.onmicrosoft.com>
Change-Id: I34d52e3799d83902e12be95d12f4036b70d721cd
diff --git a/meta-phosphor/recipes-core/os-release/os-release.bbappend b/meta-phosphor/recipes-core/os-release/os-release.bbappend
index 4c9e157..1585232 100644
--- a/meta-phosphor/recipes-core/os-release/os-release.bbappend
+++ b/meta-phosphor/recipes-core/os-release/os-release.bbappend
@@ -21,7 +21,7 @@
 BUILD_ID := "${@run_git(d, 'describe --abbrev=0')}"
 OPENBMC_TARGET_MACHINE = "${MACHINE}"
 
-OS_RELEASE_FIELDS_append = " BUILD_ID OPENBMC_TARGET_MACHINE"
+OS_RELEASE_FIELDS_append = " BUILD_ID OPENBMC_TARGET_MACHINE EXTENDED_VERSION"
 
 # Ensure the git commands run every time bitbake is invoked.
 BB_DONT_CACHE = "1"