lldpd: allow discovery protocols to be disabled from menuconfig

Signed-off-by: Michel Stam <m.stam@fugro.nl>
[jow: fixed condition for CONFIG_LLDPD_WITH_JSON]
Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43233 3c298f89-4303-0410-b956-a3cf2f4a3e73