update_payload: Remove ctypes dependency.

ctypes requires libffi.so to be installed in the system, but old
test_images don't have it, preventing "cros flash" to work there.
This patch removes the need for ctypes from common.py so cros flash
can work.

BUG=None
TEST=cros flash from R29 on link.

Change-Id: Idd0732660780081c26375a5214167b53f625e3ed
Reviewed-on: https://chromium-review.googlesource.com/277070
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/host/lib/update_payload/common.py b/host/lib/update_payload/common.py
index 573fda4..e8c7c76 100644
--- a/host/lib/update_payload/common.py
+++ b/host/lib/update_payload/common.py
@@ -6,8 +6,6 @@
 
 from __future__ import print_function
 
-import ctypes
-
 from error import PayloadError
 import update_metadata_pb2
 
@@ -15,7 +13,7 @@
 #
 # Constants.
 #
-PSEUDO_EXTENT_MARKER = ctypes.c_uint64(-1).value
+PSEUDO_EXTENT_MARKER = (1L << 64) - 1  # UINT64_MAX
 
 SIG_ASN1_HEADER = (
     '\x30\x31\x30\x0d\x06\x09\x60\x86'