mgcp: Introduce a mgcp_trunk_config enum for endpoint configs

We want to support real trunks in the MGCP code and we need to
have some better book keeping for those. Move the code around.
diff --git a/openbsc/tests/mgcp/mgcp_test.c b/openbsc/tests/mgcp/mgcp_test.c
index c039365..c63a786 100644
--- a/openbsc/tests/mgcp/mgcp_test.c
+++ b/openbsc/tests/mgcp/mgcp_test.c
@@ -47,7 +47,7 @@
 	struct msgb *inp;
 	struct msgb *msg;
 	struct mgcp_config *cfg = mgcp_config_alloc();
-	cfg->number_endpoints = 64;
+	cfg->trunk.number_endpoints = 64;
 	mgcp_endpoints_allocate(cfg);
 
 	inp = create_auep1();