base-files: add function to retrieve label MAC address

base-files: add function to retrieve label MAC address

To refer to the MAC address on a device's label, one can
specify the alias label-mac-device in the DTS which should
point to the bearer of the corresponding MAC address.

With the function get_mac_label, the user can retrieve then
retrieve this address and use it as a value that uniquely
identifies his device.

This is severely helpful for several downstream functionalities,
e.g. define MAC addresses of custom netifs or change the SSID to
be easily recognizable.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>