ueventd: let scripts provide firmware directories

Since some vendors will have firmware in mount points in
/mnt/vendor/..., we extend the ueventd script language to allow
specifying the firmware directories.

Also, move the existing 4 directories to ueventd.rc as a primary user
of this mechanism.

Bug: 111337229
Test: boot sailfish; firmwares load
Change-Id: I0854b0b786ad761e40d2332312c637610432fce2
diff --git a/init/firmware_handler.h b/init/firmware_handler.h
index e456ac4..6081511 100644
--- a/init/firmware_handler.h
+++ b/init/firmware_handler.h
@@ -17,11 +17,16 @@
 #ifndef _INIT_FIRMWARE_HANDLER_H
 #define _INIT_FIRMWARE_HANDLER_H
 
+#include <string>
+#include <vector>
+
 #include "uevent.h"
 
 namespace android {
 namespace init {
 
+extern std::vector<std::string> firmware_directories;
+
 void HandleFirmwareEvent(const Uevent& uevent);
 
 }  // namespace init