These use the raw console interface and poll. Unfortunately, the SiFive
UART puts the FIFO empty bit inside the FIFO data register, which means
that the act of checking whether a character is available also dequeues
any character from the FIFO, requiring the user to press each key twice.
However, since we configure the watermark to be 0 and, when the UART has
been grabbed for the console, we have interrupts off, we can abuse the
interrupt pending register to act as a substitute for the FIFO empty
bit.
This perhaps suggests that the console interface should move from having
rxready and getc to having getc_nonblock and getc (or make getc take a
bool), as all the places that call rxready do so to avoid blocking on
getc when there is no character available.