base-files: board.d: support dsl modems, atm bridges and pppoe protocol

This adds support for specifying the dsl modem, atm bridge configs and setting
the pppoe protocol for wan.

These additions are required to port the Lantiq target to board.d.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 47750