add support for board named RTA770W

Actually this board is the very same as the rta770bw,
where the additional 'b' within the name just
indicates 'Annex B'. The ADSL Modem itself is able
to handle both, Annex A as well as Annex B -
the loaded firmware makes the only difference

git-svn-id: svn://svn.openwrt.org/openwrt@35500 3c298f89-4303-0410-b956-a3cf2f4a3e73
fix trx checksum after first boot and add led/button definitions for boards RTA770BW and RTA770W

git-svn-id: svn://svn.openwrt.org/openwrt@35501 3c298f89-4303-0410-b956-a3cf2f4a3e73