This adds a parameter to the function stlink_open_usb and to the binary
st-flash to specify one of multiple connected stlinks.
As the identifier the iSerial of the stlink is used.
If no serial is given the function and binary behave as before.
The indentation of various st-link source files is highly inconsistent.
Reindent all source files to 4 space indentions for consistency.
I went with 4 space indentations, as it was the most common style.