vxlan: add extra config options

vxlan: add extra config options

Add config options:
  srcportmin/srcportmax : range of port numbers to use as  UDP source ports
                          to communicate to the remote VXLAN tunnel endpoint
  ageing                : lifetime in seconds of FDB entries learnt by the kernel
  maxaddress            : maximum number of FDB entries
  learning              : enable/disable entering unknown source link layer addresses
                          and IP addresses into the VXLAN device FDB.
  rsc                   : enable/disable route short circuit
  proxy                 : enable/disable ARP proxy
  l2miss                : enable/disable netlink LLADDR miss notifications
  l3miss                : enable/disable netlink IP ADDR miss notifications
  gbp                   : enable/disable the Group Policy extension

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>