lldpd: various enhancements

	- introduce "list interface" options to specify the ifaces to listen on, takes uci- or network device name
	- set the default system description to distrib @ hostname, e.g. "OpenWrt Barrier Breaker r34744 @ vbox"
	- introduce "option lldp_description" to override the automatically generated description

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34970 3c298f89-4303-0410-b956-a3cf2f4a3e73