os-release: only run bbappend on do_compile

Chasing an intermittent os-release build failure via
openbmc/openbmc#3720. Not clear if this fix will resolve
that issue but there does not appear to be any reason to
run the logic in the bbappend outside of the do_compile
step since that is the only place the OS_RELEASE_FIELDS
are utilized by the base recipe.

Change-Id: Iefaa3a16af7572f338a056115e2cabb463e74b7e
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/recipes-core/os-release/os-release.bbappend b/recipes-core/os-release/os-release.bbappend
index c010146..1c98781 100644
--- a/recipes-core/os-release/os-release.bbappend
+++ b/recipes-core/os-release/os-release.bbappend
@@ -13,7 +13,7 @@
     except:
         pass
 
-python() {
+python do_compile_prepend() {
     version_id = run_git(d, 'describe --dirty --long')
     if version_id:
         d.setVar('VERSION_ID', version_id)