Implement and use BlGetEfiPath() routine to get valid EFI path

xtldr/includes/xtldr.h

@@ -130,6 +130,11 @@ XTCDECL
 PWCHAR
 BlGetConfigValue(IN CONST PWCHAR ConfigName);
 
+XTCDECL
+EFI_STATUS
+BlGetEfiPath(IN PWCHAR SystemPath,
+             OUT PWCHAR *EfiPath);
+
 XTCDECL
 EFI_STATUS
 BlGetMemoryMap(OUT PEFI_MEMORY_MAP MemoryMap);