update_engine: Rename signing variables.

Rename 'signatures' to 'payload_signatures' so there is a clear distinction
between metadata and payload signatures and their input files.

BUG=chromium:845614
TEST=payload tryjobs
CQ-DEPEND=CL:1069530

Change-Id: Ic2e924871611af1b1f0ee2f1dfd7c6b9f52decc8
Reviewed-on: https://chromium-review.googlesource.com/1069527
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
diff --git a/scripts/brillo_update_payload b/scripts/brillo_update_payload
index 61ecadf..3802e13 100755
--- a/scripts/brillo_update_payload
+++ b/scripts/brillo_update_payload
@@ -628,7 +628,7 @@
   GENERATOR_ARGS=(
     -in_file="${FLAGS_unsigned_payload}"
     -signature_size="${FLAGS_signature_size}"
-    -signature_file="${FLAGS_payload_signature_file}"
+    -payload_signature_file="${FLAGS_payload_signature_file}"
     -metadata_signature_file="${FLAGS_metadata_signature_file}"
     -out_file="${FLAGS_payload}"
   )