Merge usb-serial-rs485 tree from libopencm3-tests repository

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+------------------------------------------------------------------------------
+README
+------------------------------------------------------------------------------
+
+current status:
+f1: frootloop3 gets corrupt data in rx path, _after_ serial rx,
+but the usb in packets (wireshark) show the corruption. (LA on serial lines is clean)
+f4: frootloop3 tests clean
+=> suspect libopecm3 usb bug?
+
+frootloop3: github.com/karlp/frootloop3
+
+
+
+This implements a USB CDC-ACM device, connected to USART2 (PA2-tx/PA3-rx)
+An interrupt driven tx ring buffer is used for usb->serial, and a similar
+interrupt driven rx ring buffer is used for serial->usb.
+Baud rates from 2400-460800 are tested with zmodem transfers in both directions.
+
+Parity not yet finished, and baudrates below 1200 need fiddling with clock prescalers.
+When it's finished, rs485 support should be available via the TC interrupt.
+
+A GPIO is toggled around USART transmissions to control an RS485 line
+transceiver, following ST's app note. The RS485 driver enable line is,
+by default, just the onboard red LEDs.
+
+The orange/blue LEDs indicate tx/rx activity, but especially for the rx line,
+you won't really see it without heavy data rates.
+
+This example is heavily based on the existing usb_cdcacm and usart demos.
+
+
+KARL - check https://github.com/dhylands/libopencm3-usb-serial/blob/master/usb.c to see if it passes the same tests or not