Fix install mbr to builder mode

install/cl_install.py

@@ -482,7 +482,8 @@ class Install(color_print):
             else:
                 prefixBoot = ""
             grubProcess = process(cmdGrubInstall,
-                              "--boot-directory=%s"%target.getBootDirectory(),
+                              "--boot-directory=%s"%pathJoin(prefixBoot,
+                              target.getBootDirectory()),
                               mbrDisk, "-f", stderr=STDOUT,envdict=os.environ)
             if grubProcess.failed():
                 raise DistributiveError(_("Failed to install the bootloader"))