Offloaded TLSSocket for BG96

Some external modems have an internal TLSSocket implementation which can be used
instead of mbedtls based TLSSocket. Using offloaded TLSSocket can result in
significantly reduced ROM usage.

Offloaded TLSSocket can be enabled by enabling "nsapi.offload-tlssocket" and the used
network stack (e.g. cellular modem's CellularStack class) must support the setsockopt's
defined in nsapi_types.h.

Compared to original mbedtls based TLSSocket, offloaded TLSSocket brings in one significant
API limitation. Offloaded TLSSocket requires setting of certificates and keys after open()
and before connect() calls, where mbedtls based TLSSocket allows setting these before open()
call.

Author: Kimmo Vaisanen

features/cellular/framework/AT/AT_CellularStack.h

@@ -102,6 +102,7 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
             started(false),
             tx_ready(false),
             rx_avail(false),
+            tls_socket(false),
             pending_bytes(0)
         {
         }
@@ -119,6 +120,7 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
         bool started; // socket has been opened on modem stack
         bool tx_ready; // socket is ready for sending on modem stack
         bool rx_avail; // socket has data for reading on modem stack
+        bool tls_socket; // socket uses modem's internal TLS socket functionality
         nsapi_size_t pending_bytes; // The number of received bytes pending
     };