6rd: Move to plugin

- Change toplevel plugins make target. Now builds all plugins under
  plugins/. (Apart from sample-plugin).
- Move sixrd code to plugins directory and make necessary changes to
  make it a plugin
- Remove 6rd hooks from IP lookup code

Change-Id: I447e92e3bee240cd8de01d0abac2e1708e8c27d1
Signed-off-by: Ole Troan <ot@cisco.com>
diff --git a/vnet/Makefile.am b/vnet/Makefile.am
index 3d9e07d..b758c7f 100644
--- a/vnet/Makefile.am
+++ b/vnet/Makefile.am
@@ -346,15 +346,11 @@
  vnet/map/map.c                                 \
  vnet/map/ip4_map.c                             \
  vnet/map/ip6_map.c                             \
- vnet/map/sixrd.c                               \
- vnet/map/ip4_sixrd.c                           \
- vnet/map/ip6_sixrd.c                           \
  vnet/map/ip4_map_t.c                           \
  vnet/map/ip6_map_t.c
 
 nobase_include_HEADERS +=                       \
- vnet/map/map.h                                 \
- vnet/map/sixrd.h
+ vnet/map/map.h
 
 ########################################
 # Tunnel protocol: gre