Cellular: Enable ePCO IE for BG96 driver

As per 3GPP TS 24.301:

If the UE supports NB-S1 mode, Non-IP PDN type, or N1 mode,
then the UE shall support the extended protocol configuration options IE.
...
If the UE supports the extended protocol configuration options IE,
then the UE shall set the ePCO bit to extended protocol configuration options supported
in the UE network capability IE of the ATTACH REQUEST message.

Author: Ari Parkkila

features/cellular/framework/targets/QUECTEL/BG96/QUECTEL_BG96_CellularNetwork.cpp

@@ -43,6 +43,12 @@ nsapi_error_t QUECTEL_BG96_CellularNetwork::set_access_technology_impl(RadioAcce
             _at.at_cmd_discard("+QCFG", "=\"nwscanseq\",030201");
             _at.at_cmd_discard("+QCFG", "=\"nwscanmode\",3,1");
             _at.at_cmd_discard("+QCFG", "=\"iotopmode\",1,1");
+#ifdef MBED_CONF_CELLULAR_OFFLOAD_DNS_QUERIES
+            // enable ePCO IE for DNS address
+            _at.cmd_start("AT+QNVFW=");
+            _at.write_string("/nv/item_files/modem/nas/nas_ciot_capability_config");
+            _at.write_string("0101", false);
+#endif
             break;
         case RAT_GSM:
         case RAT_GSM_COMPACT:

features/cellular/framework/targets/QUECTEL/BG96/QUECTEL_BG96_CellularStack.cpp

@@ -32,6 +32,9 @@ using namespace mbed;
 
 QUECTEL_BG96_CellularStack::QUECTEL_BG96_CellularStack(ATHandler &atHandler, int cid, nsapi_ip_stack_t stack_type) : AT_CellularStack(atHandler, cid, stack_type)
 #ifdef MBED_CONF_CELLULAR_OFFLOAD_DNS_QUERIES
+#if (MBED_CONF_CELLULAR_OFFLOAD_DNS_QUERIES != 1)
+#error Define cellular.offload-dns-queries to null or 1.
+#endif
     , _dns_callback(NULL), _dns_version(NSAPI_UNSPEC)
 #endif
     , _tls_sec_level(0)