extmod/modbluetooth: Re-instate optional no-ringbuf modbluetooth.

This requires that the event handlers are called from non-interrupt context
(i.e. the MicroPython scheduler).

This will allow the BLE stack (e.g. NimBLE) to run from the scheduler
rather than an IRQ like PENDSV, and therefore be able to invoke Python
callbacks directly/synchronously.  This allows writing Python BLE handlers
for events that require immediate response such as _IRQ_READ_REQUEST (which
was previous a hard IRQ) and future events relating to pairing/bonding.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/btstack/modbluetooth_btstack.c

@@ -838,15 +838,15 @@ STATIC uint16_t att_read_callback(hci_con_handle_t connection_handle, uint16_t a
         return 0;
     }
 
-    #if MICROPY_PY_BLUETOOTH_GATTS_ON_READ_CALLBACK
     // Allow Python code to override value (by using gatts_write), or deny (by returning false) the read.
+    // Note this will be a no-op if the ringbuffer implementation is being used, as the Python callback cannot
+    // be executed synchronously. This is currently always the case for btstack.
     if ((buffer == NULL) && (buffer_size == 0)) {
         if (!mp_bluetooth_gatts_on_read_request(connection_handle, att_handle)) {
             DEBUG_printf("att_read_callback: read request denied\n");
             return 0;
         }
     }
-    #endif
 
     uint16_t ret = att_read_callback_handle_blob(entry->data, entry->data_len, offset, buffer, buffer_size);
     return ret;