muge(4): add DTB blob as one more possible source of MAC address

On FDT-enabled platforms check if DTB blob has MAC address configured by
a boot loader. This information passed as a "local-mac-address" or
"mac-address" property of the device node. For USB NICs node
can be found by looking for compatibility string "usbVVV,PPP" where
VVV - vendor id (hex) and PPP - product id (hex)

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D16117