stages/grub2: greenboot support

Greenboot is the idea of automatically rolling back bad updates,
i.e. updates that do not boot successfully. The implementation
is split between the boot loader and a user space component.
The latter sets two variables `boot_counter`, which indicates
the maximum number of boot attempts and `boot_success` which
tells the boot laoder if a previous boot was successful. The
bootloader on the other hand will decrement the counter variable
and reset the success indicator one.
An implementation of the user space component for rpm-ostree is
called `greenboot`.

test/data/manifests/fedora-ostree-image.json

@@ -942,7 +942,8 @@
               "install": true
             },
             "legacy": "i386-pc",
-            "write_defaults": false
+            "write_defaults": false,
+            "greenboot": true
           }
         }
       ]