nhrpd: cache config may disappear if iface not present at startup

nhrpd: cache config may disappear if iface not present at startup

When interface not present at config time, store separately the list of
config parameters. Then, when interface is ready and an address has been configured, the nbma setting is done. Reversely, when interface disappears,
there is no need to keep the maps present, then keep only the configuration.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>