flashtool.py: Added support for sending commands to enter the bootloader

[Lebensgefahr]

for both virtual and physical serial devices.

I added commands to reboot into the bootloader for both virtual and physical serial devices. I slightly changed the logic: with the -r parameter, the script only sends the command to enter the bootloader and exits, while without this option, no attempts to enter the bootloader are made. It seemed to me that this approach reduces the chances of flashing something incorrectly and allows you to verify the connection to the bootloader beforehand using the -q option.