Consolidate the codes that handle reboot/shutdown.

Test: Choose `Reboot system now`, `Power off`, `Reboot to bootloader`
      from recovery UI respectively.
Test: `adb reboot recovery` while under sideload mode.
Change-Id: I0f3d55b80b472178ea4f6970b29cd9df0778b639
diff --git a/recovery_ui/ui.cpp b/recovery_ui/ui.cpp
index b7107ff..7ea9307 100644
--- a/recovery_ui/ui.cpp
+++ b/recovery_ui/ui.cpp
@@ -375,7 +375,7 @@
 
       case RecoveryUI::REBOOT:
         if (reboot_enabled) {
-          reboot("reboot,");
+          Reboot("");
           while (true) {
             pause();
           }