moveconfig: Use a function to read files

moveconfig: Use a function to read files

At present there is quite a bit of ad-hoc code reading from files. The
most common case is to read the file as lines. Put it in a function and
set the unicode encoding correctly.

Avoid writing back to a file when there are obviously no changes as this
speeds things up slightly.

Signed-off-by: Simon Glass <sjg@chromium.org>