usb转串口模块驱动添加编译

有一些常用的usb转串口模块，ft232，ch340等，驱动默认是没有打开。

menuconfig选上编译：

ft232(drivers/usb/serial/ftdi_sio.c)：

Device Drivers  --->
      [*] USB support  --->
        <*>   USB Serial Converter support  --->
        <*>   USB FTDI Single Port Serial Driver 

ch340(drivers/usb/serial/ch341.c):

Device Drivers  --->
      [*] USB support  --->
        <*>   USB Serial Converter support  --->
        <*>   USB Winchiphead CH341 Single Port Serial Driver