Make SHA library accessible to calling firmware

And add a vb2_digest_buffer() call which produces the hash of a buffer
all in a single function call.  That function actually already
existed, but was in a unit test file rather than in the library
itself.  It's a small function, so adding it won't increase the size
of the library significantly - or at all, on platforms which compile
with -ffunction-sections.

This allows coreboot to reuse this SHA library for hashing CBFS
entries and file data.  All it has to do is #define
NEED_VB2_SHA_LIBRARY and then #include "vb2_api.h".

BUG=chromium:482652
BRANCH=none
TEST=make -j runtests

Change-Id: Ice2d0929324b58b2665f3989b5b887225f6ef61e
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/269523
Reviewed-by: Julius Werner <jwerner@chromium.org>

firmware/include/vb2_api.h

@@ -6,6 +6,10 @@
 /* APIs between calling firmware and vboot_reference
  *
  * DO NOT INCLUDE THE HEADERS BELOW DIRECTLY!  ONLY INCLUDE THIS FILE!
+ *
+ * Using vb2api.h as the single point of contact between calling firmware and
+ * vboot allows subsequent refactoring of vboot (renaming of headers, etc.)
+ * without churning other projects' source code.
  */
 
 #ifndef VBOOT_VB2_API_H_
@@ -14,6 +18,11 @@
 /* Standard APIs */
 #include "../2lib/include/2api.h"
 
+/* SHA library */
+#ifdef NEED_VB2_SHA_LIBRARY
+#include "../2lib/include/2sha.h"
+#endif
+
 /*
  * Coreboot should not need access to vboot2 internals.  But right now it does.
  * At least this forces it to do so through a relatively narrow hole so vboot2