Staging: batman-adv: Allow the MAC address to be set

Some embedded devices have very limited sources of entropy for the
random number generator. It has been observed that the random MAC
address on the interface bat0 is not always random. When testing with
a collection of identical hardware, sometimes the bat0 device the same
MAC address on multiple devices, causing mayhem. This patch allows the
MAC address to be set by the user.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>