DFU: Move "Override DFU" (force DFU) code to board-specific section

Each board can define its own conditions on which the controller should
boot into DFU mode rather than normal application mode.  Let's move the
"UART loopback jumper" to QMOD specific part.  For SIMtrace we have an
actual button and can use that in a future patch.

firmware/libboard/qmod/source/board_qmod.c

@@ -249,3 +249,39 @@ void board_main_top(void)
 	card_present_init();
 #endif
 }
+
+static int uart_has_loopback_jumper(void)
+{
+	unsigned int i;
+	const Pin uart_loopback_pins[] = {
+		{PIO_PA9A_URXD0, PIOA, ID_PIOA, PIO_INPUT, PIO_DEFAULT},
+		{PIO_PA10A_UTXD0, PIOA, ID_PIOA, PIO_OUTPUT_0, PIO_DEFAULT}
+	};
+
+	/* Configure UART pins as I/O */
+	PIO_Configure(uart_loopback_pins, PIO_LISTSIZE(uart_loopback_pins));
+
+	for (i = 0; i < 10; i++) {
+		/* Set TxD high; abort if RxD doesn't go high either */
+		PIO_Set(&uart_loopback_pins[1]);
+		if (!PIO_Get(&uart_loopback_pins[0]))
+			return 0;
+		/* Set TxD low, abort if RxD doesn't go low either */
+		PIO_Clear(&uart_loopback_pins[1]);
+		if (PIO_Get(&uart_loopback_pins[0]))
+			return 0;
+	}
+	/* if we reached here, RxD always follows TxD and thus a
+	 * loopback jumper has been placed on RxD/TxD, and we will boot
+	 * into DFU unconditionally */
+	return 1;
+}
+
+int board_override_enter_dfu(void)
+{
+	/* If the loopback jumper is set, we enter DFU mode */
+	if (uart_has_loopback_jumper())
+		return 1;
+
+	return 0;
+}