rampayload: Add the linuxcheck payload

The i386.c file uses standard 3f8 UART for some simple diagnostic
prints, and the libpayload console otherwise.

This payload was used to debug Linux as a rampayload and was very helpful
for that work.

Change-Id: I1cce5528780cd825fd91a88137fa70abd9f218e7
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/28600
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/linuxcheck/i386.c

@@ -0,0 +1,48 @@
+/*
+ * This file is part of the coreinfo project.
+ *
+ * Copyright (C) 2018 Google Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <libpayload-config.h>
+#include <libpayload.h>
+#include "linuxcheck.h"
+
+void buts(char *s)
+{
+	int i;
+	for (i = 0; i < strlen(s); i++)
+		outb(s[i], 0x3f8);
+}
+
+void hex4(u8 c)
+{
+	static char *hex = "0123456789abcdef";
+	outb(hex[c & 0xf], 0x3f8);
+}
+
+void hex8(u8 c)
+{
+	hex4(c >> 4);
+	hex4(c);
+}
+
+void hex16(u16 c)
+{
+	hex8((u8)(c >> 8));
+	hex8((u8)c);
+}
+void hex32(u32 c)
+{
+	hex16((u16)(c >> 16));
+	hex16((u16)c);
+}