sifive_uart: Fix input character dropping in ddb and at a mountroot prompt

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.

Reviewed by:	kp, philip
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D31025

sys/riscv/sifive/sifive_uart.c

@@ -137,9 +137,15 @@ sfuart_putc(struct uart_bas *bas, int c)
 static int
 sfuart_rxready(struct uart_bas *bas)
 {
-
-	return ((uart_getreg(bas, SFUART_RXDATA) &
-	    SFUART_RXDATA_EMPTY) == 0);
+	/*
+	 * Unfortunately the FIFO empty flag is in the FIFO data register so
+	 * reading it would dequeue the character. Instead, rely on the fact
+	 * we've configured the watermark to be 0 and that interrupts are off
+	 * when using the low-level console function, and read the interrupt
+	 * pending state instead.
+	 */
+	return ((uart_getreg(bas, SFUART_IRQ_PENDING) &
+	    SFUART_IRQ_PENDING_RXQM) != 0);
 }
 
 static int