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/linuxcheck.h

@@ -0,0 +1,22 @@
+/*
+ * This file is part of the coreboot 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.
+ */
+
+/* buts is a programmed IO byte puts, which you need to write for all platforms. */
+void buts(char *s);
+
+void hex4(u8 c);
+void hex8(u8 c);
+void hex16(u16 c);
+void hex32(u32 c);