paycheck: use base64 encoding for SHA256 hashes throughout

Turns out we were using base64 when adding SHA256 data to a payload
report, but intermittently using hex encoding in error messages. This is
now fixed.

BUG=None
TEST=No mention of hex encoding in the code
TEST=gpylinted correctly
TEST=Passes unit tests

Change-Id: Id2dc2fcd154d4647cc1a076579dde7a789c09e40
Reviewed-on: https://gerrit.chromium.org/gerrit/50104
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
diff --git a/scripts/update_payload/applier.py b/scripts/update_payload/applier.py
index 3b7b1a6..cd9fd06 100644
--- a/scripts/update_payload/applier.py
+++ b/scripts/update_payload/applier.py
@@ -62,8 +62,8 @@
   actual_hash = hasher.digest()
   if actual_hash != expected_hash:
     raise PayloadError('%s hash (%s) not as expected (%s)' %
-                       (name, actual_hash.encode('hex'),
-                        expected_hash.encode('hex')))
+                       (name, common.FormatSha256(actual_hash),
+                        common.FormatSha256(expected_hash)))
 
 
 def _ReadExtents(file_obj, extents, block_size, max_length=-1):