ubifs: Create hash for default LPT

During creation of the default filesystem on an empty flash the default
LPT is created. With this patch a hash over the default LPT is
calculated which can be added to the default filesystems master node.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>