Change os_install_mbr fill method.

If loaded system is livecd, then mbr by default is
/boot,/ device.

pym/cl_fill_install.py

@@ -1112,7 +1112,8 @@ class fillVars(object, glob_attr):
         return res[0][0]
 
     def get_os_install_mbr(self):
-        if self.Get('os_install_root_type') == "flash":
+        if self.Get('os_install_root_type') == "flash" or \
+            self.Get('os_root_type') == "livecd":
             rootdev = self.Get('os_install_root_dev')
             device = filter(lambda x:x in rootdev,
                                 self.Get('os_device_dev'))