STM32F4 CDC UART support + other CDC changes

This should really be more than one commit, but here it goes anyway:

- added STM32F4 CDC UART support. For some reason, it seems to work only
on the OTG_FS interface, the OTG_HS interface enumerates it, but
doesn't seem to send/receive any data.
- removed the "platform interface" functions for CDC UART. That was
never really a platform interface, just a couple of functions reading
and writing data from/to a CDC UART. Now each backend takes care of this
as a special case in its platform_uart_xxx functions.
- added buffering support for CDC UARTs
- added uart.CDC to the uart module, so the CDC uart can be used directly
from Lua.
- stm32f4discovery now defaults to using the CDC, since it doesn't have
a dedicated UART connector.