cp210x wierd behaviour

[mczerski]

I couldn't get my cp210x based usb uart (like this) to flash bk7231n mcu on linux. The mcu was in reset the whole time during flash. After some digging I found out that my usb adapter asserts rts signal (makes it low) just after open and stays that way. I solved my problem by adding setting self.serial.rts = False in BK7231Serial init. I guess that it should be fixed in cp210x driver but ensuring rts is False afer opening port does not hurt much and allows to use cp210x based uarts with this project.