nautilus: Support tristate board id strapping pin

With 3 binary strapping pins, we only have 7 available board ids:
000, 001, 010, 011, 100, 101, 110, 111.

Let's make the MSB of board id tristate. So we can have 4 more
board ids to use:
Z00, Z01, Z10, Z11.

BUG=b:77731277
BRANCH=poppy
TEST=build nautilus

Change-Id: I7aebb89437d2ccb9eea6c477155b25d964983232
Signed-off-by: Philip Chen <philipchen@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1000875
Commit-Ready: Philip Chen <philipchen@chromium.org>
Tested-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>

board/nautilus/board.c

@@ -580,6 +580,34 @@ void board_hibernate(void)
 		;
 }
 
+int board_get_version(void)
+{
+	static int ver = -1;
+	uint8_t id3;
+
+	if (ver != -1)
+		return ver;
+
+	ver = 0;
+
+	/* First 2 strappings are binary. */
+	if (gpio_get_level(GPIO_BOARD_VERSION1))
+		ver |= 0x01;
+	if (gpio_get_level(GPIO_BOARD_VERSION2))
+		ver |= 0x02;
+
+	/*
+	 * The 3rd strapping pin is tristate.
+	 * id3 = 2 if Hi-Z, id3 = 1 if high, and id3 = 0 if low.
+	 */
+	id3 = gpio_get_ternary(GPIO_BOARD_VERSION3);
+	ver |= id3 * 0x04;
+
+	CPRINTS("Board ID = %d", ver);
+
+	return ver;
+}
+
 /* Lid Sensor mutex */
 static struct mutex g_lid_mutex;
 static struct mutex g_base_mutex;

board/nautilus/board.h

@@ -18,6 +18,7 @@
 #define CONFIG_ADC
 #define CONFIG_BACKLIGHT_LID
 #define CONFIG_BOARD_VERSION
+#define CONFIG_BOARD_SPECIFIC_VERSION
 #define CONFIG_BOARD_FORCE_RESET_PIN
 #define CONFIG_CASE_CLOSED_DEBUG_EXTERNAL
 #define CONFIG_DPTF