tools/pyboard.py: Change logic for when raw ">" prompt is parsed.
In raw REPL ">" indicates the prompt. We originally read this character upon entering the raw REPL, and after reading the last bit of the output. This patch changes the logic so the ">" is read only just before trying to send the next command. To make this work (and as an added feature) the input buffer is now flushed upon entering raw REPL. The main reason for this change is so that pyboard.py recognises the EOF when sys.exit() is called on the pyboard. Ie, if you run pyboard.py with a script that calls sys.exit(), then pyboard.py will exit after the sys.exit() is called.