binman: Put fake files in a subdirectory

binman: Put fake files in a subdirectory

At present fake files from a previous build appear to be real files for
a subsequent build, since they sit in the output directory.

This can cause problems, since binman may need to parse the file, e.g.
with the Intel description.bin files.

Fix this by putting them in a 'binman-fake' subdirectory. Keep a track
of the fake filename so we only create it once. Subsequent builds will
still see that the file is missing and mark it as fake.

Update a few tests to check the behaviour.

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