sandbox: fs: Add support for saving files to host filesystem

This allows write of files from the host filesystem in sandbox. There is
currently no concept of overwriting the file and removing its existing
contents - all writing is done on top of what is there. This means that
writing 10 bytes to the start of a 1KB file will only update those 10
bytes, not truncate the file to 10 byte slong.

If the file does not exist it is created.

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