Use ALPHA value received in SI13 from PCUIF

The old VTY command is marked as deprecated and still overrides the use
in case it's used.

Related: SYS#5358
Depends: libosmocore.git Change-Id I74fb0a3afc1ac4aadbfc609b882d929401f790eb
Depends: osmo-bsc.git Change-Id I8b97ea11bad5fe05f2f634945b5703ee9abde81d
Change-Id: I46f2a955b157a409055fca7fb917dc4f75482426
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index b6d6786..7753557 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -42,6 +42,7 @@
 #include <osmocom/gprs/gprs_ns2.h>
 #include <osmocom/gsm/l1sap.h>
 #include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/gsm/gsm48_rest_octets.h>
 #include <osmocom/gsm/sysinfo.h>
 }
 
@@ -291,6 +292,7 @@
 
 static int pcu_rx_data_ind_bcch(struct gprs_rlcmac_bts *bts, uint8_t *data, uint8_t len)
 {
+	const uint8_t *si_ro;
 	switch (len) {
 	case 0:
 		/* Due to historical reasons also accept a completely empty message as
@@ -337,6 +339,9 @@
 		case GSM48_MT_RR_SYSINFO_13:
 			memcpy(bts->si13, data, GSM_MACBLOCK_LEN);
 			bts->si13_is_set = true;
+			si_ro = ((struct gsm48_system_information_type_13*)data)->rest_octets;
+			if (osmo_gsm48_rest_octets_si13_decode(&bts->si31_ro_decoded, si_ro) < 0)
+				LOGP(DPCU, LOGL_ERROR, "Error decoding SI13\n");
 			break;
 		default:
 			LOGP(DL1IF, LOGL_ERROR,