[abis] Add ACK signal and send it on SET NVATTR ACK

* Add the signal definition to signal.h
* Dispatch the signal from abis_nm.c
* Handle it in ipaccess-config.c and say we are done with work
diff --git a/openbsc/src/ipaccess/ipaccess-config.c b/openbsc/src/ipaccess/ipaccess-config.c
index 016459b..3b81c2a 100644
--- a/openbsc/src/ipaccess/ipaccess-config.c
+++ b/openbsc/src/ipaccess/ipaccess-config.c
@@ -83,6 +83,16 @@
 	return 0;
 }
 
+static int ipacc_msg_ack(u_int8_t mt)
+{
+	if (sw_load_state == 1) {
+		fprintf(stderr, "The new software is activaed.\n");
+		exit(0);
+	}
+
+	return 0;
+}
+
 struct ipacc_ferr_elem {
 	int16_t freq_err;
 	u_int8_t freq_qual;
@@ -170,6 +180,9 @@
 	case S_NM_IPACC_NACK:
 		msg_type = signal_data;
 		return ipacc_msg_nack(*msg_type);
+	case S_NM_IPACC_ACK:
+		msg_type = signal_data;
+		return ipacc_msg_ack(*msg_type);
 	case S_NM_TEST_REP:
 		return test_rep(signal_data);
 	default: