Implement BlEnterFirmwareSetup() routine

xtldr/includes/xtldr.h

@@ -95,6 +95,10 @@ XTCDECL
 VOID
 BlEnableConsoleCursor();
 
+XTCDECL
+EFI_STATUS
+BlEnterFirmwareSetup();
+
 XTCDECL
 EFI_STATUS
 BlEnumerateBlockDevices();