init: Support custom shutdown actions

We have been seeing panics and errors during shutdown sequence in
some vendor's platform, and it is required to disable error handling
during shutdown.

This CL separates the shutdown request to execute another "shutdown"
trigger at the beginning of shutdown stage. And vendor can use this
trigger to add custom commands needed for shutting down gracefully.

Bug: 38203024
Bug: 62084631
Test: device reboot/shutdown
Change-Id: I3fac4ed59f06667d86e477ee55ed391cf113717f
(cherry picked from commit eeab491efd8f456324f88e444f228b1016712e45)
diff --git a/init/util.cpp b/init/util.cpp
index 75f81b9..dfdfeb4 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -367,6 +367,7 @@
 
 void panic() {
     LOG(ERROR) << "panic: rebooting to bootloader";
+    // Do not queue "shutdown" trigger since we want to shutdown immediately
     DoReboot(ANDROID_RB_RESTART2, "reboot", "bootloader", false);
 }