grub2: Import greenboot configs

Copied from https://github.com/osbuild/osbuild/blob/4e5de3808e2bf56e1060937c4d5e1796df441fa4/stages/org.osbuild.grub2#L280

Signed-off-by: Colin Walters <walters@verbum.org>

src/grub2/grub-static-greenboot.cfg

@@ -0,0 +1,19 @@
+# greenboot support, aka boot counter and boot success reporting
+insmod increment
+# Check if boot_counter exists and boot_success=0 to activate this behaviour.
+if [ -n "${boot_counter}" -a "${boot_success}" = "0" ]; then
+  # if countdown has ended, choose to boot rollback deployment,
+  # i.e. default=1 on OSTree-based systems.
+  if  [ "${boot_counter}" = "0" -o "${boot_counter}" = "-1" ]; then
+    set default=1
+    set boot_counter=-1
+  # otherwise decrement boot_counter
+  else
+    decrement boot_counter
+  fi
+  save_env boot_counter
+fi
+
+# Reset boot_success for current boot
+set boot_success=0
+save_env boot_success