ath79: fix WNDR3800 support

Fixes for WNDR3800 DTS:
* fix bootcmd
* name art partition to reference it for MACs
* format firmware partition's address similarly as others
* make u-boot-env read-only like in ar71xx
* read wan and lan MACs from art
* fix LED and button gpio values to match ar71xx
  (orange/green powerLED, reset button)
* enable power LED in dts by alias led-status
* define USB LED triggers and connect to USB port
* change wifi button to KEY_RFKILL like in ar71xx

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>