[PATCH RFC] Overlayfs mode support

This dramatical reduce startup time on ssd and hdd,
created for prevent useless copy browser data from ssd to tmpfs
If you want i can delete Signed-off-by;

From 3.18 kernel, linux has support overlayfs that cover AUFS and UnionFS functionality.
Ported from previous https://github.com/graysky2/profile-sync-daemon/pull/71

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>

Please test and review.