purgatory/sha256: provide implementation of sha256 in purgaotory context

Next two patches provide code for purgatory.  This is a code which does
not link against the kernel and runs stand alone.  This code runs between
two kernels.  One of the primary purpose of this code is to verify the
digest of newly loaded kernel and making sure it matches the digest
computed at kernel load time.

We use sha256 for calculating digest of kexec segmetns.  Purgatory can't
use stanard crypto API as that API is not available in purgatory context.

Hence, I have copied code from crypto/sha256_generic.c and compiled it
with purgaotry code so that it could be used.  I could not #include
sha256_generic.c file here as some of the function signature requiered
little tweaking.  Original functions work with crypto API but these ones
don't

So instead of doing #include on sha256_generic.c I just copied relevant
portions of code into arch/x86/purgatory/sha256.c.  Now we shouldn't have
to touch this code at all.  Do let me know if there are better ways to
handle it.

This patch does not enable compiling of this code.  That happens in next
patch.  I wanted to highlight this change in a separate patch for easy
review.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Greg Kroah-Hartman <greg@kroah.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: WANG Chao <chaowang@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/purgatory/sha256.h

+/*
+ *  Copyright (C) 2014 Red Hat Inc.
+ *
+ *  Author: Vivek Goyal <vgoyal@redhat.com>
+ *
+ * This source code is licensed under the GNU General Public License,
+ * Version 2.  See the file COPYING for more details.
+ */
+
+#ifndef SHA256_H
+#define SHA256_H
+
+
+#include <linux/types.h>
+#include <crypto/sha.h>
+
+extern int sha256_init(struct sha256_state *sctx);
+extern int sha256_update(struct sha256_state *sctx, const u8 *input,
+				unsigned int length);
+extern int sha256_final(struct sha256_state *sctx, u8 *hash);
+
+#endif /* SHA256_H */