src: port openBSC over libosmo-abis

This is a big patch that ports openBSC over libosmo-abis.
Sorry, the changes that are included here are all dependent
of libosmo-abis, splitting them into smaller pieces would
leave the repository in some intermediate state, which is
not desired.

The main changes are:

- The directory libabis/ has been removed as it now lives in
  libosmo-abis.

- new configuration file format for nanoBTS and HSL femto, we
  need to define the virtual e1_line and attach it to the OML
  link.

- all the existing BTS drivers (nanoBTS, hsl femto, Nokia site,
  BS11 and rbs2000) now use the new libosmo-abis framework.

- use r232 input driver available in libosmo-abis for bs11_config.

- use ipa_msg_recv instead of old ipaccess_read_msg function.

- delete definition of gsm_e1_subslot and input_signal_data.
  These structures now lives in libosmo-abis.

Most of this patch are deletions of libabis/ which has been
moved to libosmo-abis.

This patch also modifies openBSC to use all the new definitions
available in libosmocore and libosmo-abis. In order to do that,
we have replaced the following:

- DINP, DMI, DMIB and DMUX by their respective DL* correspondences.
- SS_GLOBAL by SS_L_GLOBAL
- SS_INPUT by SS_L_INPUT
- S_GLOBAL_SHUTDOWN by S_L_GLOBAL_SHUTDOWN
- SS_INPUT by SS_L_INPUT
- S_INP_* by S_L_INP_* sub-signals
- E1INP_NODE by L_E1INP_NODE vty node

This patch has been tested with:
- one nanoBTS
- the HSL femto with the examples available under libosmo-abis
- BS11 with both dahdi and misdn drivers.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index bbbeead..80d6e77 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -32,6 +32,8 @@
 
 #include <osmocom/sccp/sccp.h>
 
+#include <osmocom/abis/ipa.h>
+
 #include <sys/socket.h>
 #include <string.h>
 #include <unistd.h>
@@ -97,12 +99,13 @@
 
 static int ussd_read_cb(struct osmo_fd *bfd)
 {
-	int error;
 	struct bsc_nat_ussd_con *conn = bfd->data;
-	struct msgb *msg = ipaccess_read_msg(bfd, &error);
+	struct msgb *msg;
 	struct ipaccess_head *hh;
+	int ret;
 
-	if (!msg) {
+	ret = ipa_msg_recv(bfd->fd, &msg);
+	if (ret <= 0) {
 		LOGP(DNAT, LOGL_ERROR, "USSD Connection was lost.\n");
 		bsc_nat_ussd_destroy(conn);
 		return -1;