brcm63xx: populate /tmp/sysinfo

Take full board name from devicetree, if present. Provide a method
for retrieving the board name.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44833 3c298f89-4303-0410-b956-a3cf2f4a3e73