Refactoring uart interface (#341)

* uart: Major cleanups
- Adds documentation to the uart API.
- Fixes a buffer overflow issue in `uart_receive`, where the maximum parameter was ignored.
- Splits the maximum length and bytes recieved variables in `uart_receive`.
- Downsizes the receive buffer to the minimum required, saving 16MiB of RAM at runtime.
- Refactors the POSIX and Win32 implementations of uart into separate files.
- Removes the unused `uart_{get,set}_parity` functions, which were not implemented on Win32.

uart/README.md

@@ -0,0 +1,13 @@
+# uart
+
+This contains functionality for talking to UART/Serial devices on different platforms. The official client will build either `uart_posix.c` and `uart_win32.c`.  Build targets for these files are contained in `client/Makefile`.
+
+If you want to implement support for other platforms, you need to implement the methods provided in `uart.h`.
+
+## Implementing a new driver
+
+Each driver is called with a string, typically containing a path or other reference to a serial port on the host.  The methods outlined in `uart.h` need to be implemented.
+
+The hardware uses `common/usb_cdc.c` to implement a USB CDC endpoint exposed by the Atmel MCU.
+
+