A DFU device must be in the idle state before it can be programmed, and
this requires either clearing the status or aborting, depending on its
current state. Code is added to do this. And the USB transfer size is now
automatically detected so devices with a size less than 2048 bytes work
correctly.
There appears to be an issue on Windows with CPython >= 3.6,
sys.stdout.flush() raises an exception:
OSError: [WinError 87] The parameter is incorrect
It works fine to just catch and ignore the error on the flush line. Tested
on Windows 10 x64 1803 (Build 17134.228), Python 3.6.4 amd64.
The ST DFU bootloader supports a transfer size up to 2048 bytes, so send
that much data on each download (to device) packet. This almost halves
total download time.
