Reopen temporary system image to avoid stale data.

NamedTemporaryFile's aggressive caching behavior can cause an issue
where changes made by another process aren't visible even after the
fseek() below or a flush(). To avoid this, simply open the file
again and read from the fresh version.

This fixes an issue where verity metadata written by append2simg
doesn't become visible to img_from_target_files.

Change-Id: I291fb3a95d5b532218ac6205ecc9e9b4f3a36bd4