binman: Write fake blobs to the output directory

At present binman writes fake blobs to the current directory. This is not
very helpful, since the files serve no useful purpose once binman has
finished. They clutter up the source directory and affect future runs,
since the files in the current directory are often used in preference to
those in the board directory.

To avoid these problems, write them to the output directory instead.

Move the file-creation code to the Entry base class, so it can be used by
any entry type that needs it. This is required since some entry types,
such as Entry_blob_ext_list, are not subclasses of Entry_blob.

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