AU: Handle firmware update failure when booted from FW slot B.

postinstall_runner_action.cc

Index: postinstall_runner_action.cc
diff --git a/postinstall_runner_action.cc b/postinstall_runner_action.cc
index 80c28c955b73793a78e77516eb8fe17f9eb0e254..2c8b943c3c986a6299cfc8657b447cfc92211806 100644
--- a/postinstall_runner_action.cc
+++ b/postinstall_runner_action.cc
@@ -65,6 +65,12 @@ void PostinstallRunnerAction::CompletePostinstall(int return_code) {
   ScopedTempUnmounter temp_unmounter(temp_rootfs_dir_);
   if (return_code != 0) {
     LOG(ERROR) << "Postinst command failed with code: " << return_code;
+    if (return_code == 2) {
+      // This special return code means that we tried to update firmware,
+      // but couldn't because we booted from FW B, and we need to reboot
+      // to get back to FW A.
+      completer.set_code(kActionCodePostinstallBootedFromFirmwareB);
+    }
     return;
   }
   if (HasOutputPipe()) {