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/nimble/modbluetooth_nimble.c

@@ -586,12 +586,12 @@ static int characteristic_access_cb(uint16_t conn_handle, uint16_t value_handle,
     switch (ctxt->op) {
         case BLE_GATT_ACCESS_OP_READ_CHR:
         case BLE_GATT_ACCESS_OP_READ_DSC:
-            #if MICROPY_PY_BLUETOOTH_GATTS_ON_READ_CALLBACK
             // Allow Python code to override (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 (i.e. the stack isn't
+            // run in the scheduler). The ringbuffer is not used on STM32 and Unix-H4 only.
             if (!mp_bluetooth_gatts_on_read_request(conn_handle, value_handle)) {
                 return BLE_ATT_ERR_READ_NOT_PERMITTED;
             }
-            #endif
 
             entry = mp_bluetooth_gatts_db_lookup(MP_STATE_PORT(bluetooth_nimble_root_pointers)->gatts_db, value_handle);
             if (!entry) {