luci-base: implement further ubus calls
luci-base: implement further ubus calls

Implement new ubus rpcd backend calls for later use in the frontend:

 - netdevs: dump information about Linux network devices present
 - boardjson: dump /etc/board.json if present
 - offload_support: query whether netfilter offloading is supported

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

openwrt/luci,lbthomsen/openwrt-luci,hnyman/luci,nmav/luci,nmav/luci,artynet/luci,lbthomsen/openwrt-luci,Noltari/luci,Noltari/luci,Noltari/luci,hnyman/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,Noltari/luci,openwrt/luci,openwrt/luci,rogerpueyo/luci,Noltari/luci,artynet/luci,nmav/luci,lbthomsen/openwrt-luci,openwrt/luci,lbthomsen/openwrt-luci,nmav/luci,nmav/luci,Noltari/luci,tobiaswaldvogel/luci,artynet/luci,artynet/luci,rogerpueyo/luci,openwrt/luci,hnyman/luci,tobiaswaldvogel/luci,hnyman/luci,tobiaswaldvogel/luci,nmav/luci,rogerpueyo/luci,lbthomsen/openwrt-luci,artynet/luci,hnyman/luci,Noltari/luci,hnyman/luci,artynet/luci,nmav/luci,nmav/luci,openwrt/luci,rogerpueyo/luci,artynet/luci,tobiaswaldvogel/luci,tobiaswaldvogel/luci,rogerpueyo/luci,openwrt/luci,tobiaswaldvogel/luci,artynet/luci,nmav/luci,hnyman/luci,lbthomsen/openwrt-luci,tobiaswaldvogel/luci,hnyman/luci,rogerpueyo/luci,tobiaswaldvogel/luci,artynet/luci,Noltari/luci,Noltari/luci,rogerpueyo/luci,openwrt/luci,lbthomsen/openwrt-luci