If the device is not connected over USB CDC to a host then all output to
the CDC (eg initial boot messages) is written to the CDC TX buffer with
wrapping, so that the most recent data is retained when the USB CDC is
eventually connected (eg so the REPL banner is displayed upon connection).
This commit fixes a bug in this behaviour, which was likely introduced in
e4fcd216e02eef0b389c84ecd67be3114aac0a5d, where the initial data in the CDC
TX buffer is repeated multiple times on first connection of the device to
the host.
Signed-off-by: Damien George <damien@micropython.org>
581d43b774