Add connection status override for CellularContext

Add override for the virtual method get_connection_status() inherited
from NetworkInterface. The method in the base class returns
NSAPI_STATUS_ERROR_UNSUPPORTED. The CellularContext has the member
_connection_status, which means that we could return this.

Author: oscarh

features/cellular/framework/AT/AT_CellularContext.cpp

@@ -147,6 +147,11 @@ nsapi_error_t AT_CellularContext::check_operation(nsapi_error_t err, ContextOper
     return err;
 }
 
+nsapi_connection_status_t AT_CellularContext::get_connection_status() const
+{
+    return _connect_status;
+}
+
 uint32_t AT_CellularContext::get_timeout_for_operation(ContextOperation op) const
 {
     uint32_t timeout = NETWORK_TIMEOUT; // default timeout is 30 minutes as registration and attach may take time

features/cellular/framework/AT/AT_CellularContext.h

@@ -37,6 +37,7 @@ class AT_CellularContext : public CellularContext, public AT_CellularBase {
     virtual void attach(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
     virtual nsapi_error_t connect();
     virtual nsapi_error_t disconnect();
+    virtual nsapi_connection_status_t get_connection_status() const;
     virtual bool is_connected();
     // from CellularBase
     virtual void set_plmn(const char *plmn);