gbp: add anonymous l3-out external interfaces

So far, GBP l3-out packets classification & policy relied on programmed
EP. All traffic to/from l3-out must go through a known EP.
This patch introduces a new feature where l3-out next-hops are only
known by their subnets (l3-out prefixes). As there are no longer known
EPs to program, an interface must be configured as external anonymous
l3-out. Packets classification & policy on this interface will rely on
the external subnets programmed in the BD VRF.
Note that contrary to all other interfaces in a GBP BD, external
anonymous l3-out interfaces have BD L2 learning turned on and rely on
ARP/ND.

Type: feature

Change-Id: Ieedb29dff4e967d08c4301e82d06bff450a63e5f
Signed-off-by: Benoît Ganne <bganne@cisco.com>
diff --git a/src/vnet/l2/l2_output.h b/src/vnet/l2/l2_output.h
index 74d2829..1cc1e73 100644
--- a/src/vnet/l2/l2_output.h
+++ b/src/vnet/l2/l2_output.h
@@ -81,6 +81,7 @@
 #define foreach_l2output_feat \
  _(OUTPUT,            "interface-output")           \
  _(SPAN,              "span-l2-output")             \
+ _(GBP_POLICY_LPM,    "gbp-policy-lpm")            \
  _(GBP_POLICY_PORT,   "gbp-policy-port")            \
  _(GBP_POLICY_MAC,    "gbp-policy-mac")             \
  _(CFM,               "feature-bitmap-drop")        \