MODBUS NOT USABLE WITH USB SERIAL

```
Description: Modbus can be used with USB serial, however, if the USB
serial connection is lost, Modbus will hang in an infinite
loop.

This is a problem in the handling of select() and read()
and could probably resolved by studying the Modbus error
handling.

A more USB-friendly solution would be to: (1) Re-connect and
(2) re-open the serial drivers. That is what is done is NSH.
When the serial USB device is removed, this terminates the
session and NSH will then try to re-open the USB device. See
the function nsh_waitusbready() in the file
apps/nshlib/nsh_usbconsole.c. When the USB serial is
reconnected the open() in the function will succeed and a new
session will be started.
Status: Open
Priority: Low. This is really an enhancement request: Modbus was never
designed to work with removable serial devices.
```