K64F serial_readable gives misleading result after overrun error

[infinnovation-dev]

Using e.g. UART2 which has no FIFO, an overrun can cause the following unwelcome behaviour:

    if (serial_readable(s)) {
        c = serial_getc(s);  // Hangs forever if no further character received
        ...

When serial_readable detects an overrun error, it calls UART_ClearStatusFlags; this reads S1 and D, which as well as clearing the overrun flag also clears the RDRF flag. serial_getc now loops calling serial_readable which now returns false.

[ciarmcom]

ARM Internal Ref: IOTMORF-161