update.py: DRY up using functools.partial()

We call _copy_pyfile() a bunch of times with the same base and
dest_dir arguments. Clean that up using functools.partial().

Change-Id: I22a3e748f2e138b17a6b99d90ec9eca72698f6e7
Merge "update.py: DRY up using functools.partial()"