meta-yocto-bsp has a bbappend for netbase which includes the interfaces file customized for beagleboard, but 'interfaces' is not part of netbase but of the init-ifupdown package, so this should be done useing init-ifupdown bbappend.
http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=c97deb11ee0833fb309d78a4c88cad1cab74c37a