mkimage: Set up a file size parameter and keep it updated

Some functions called by mkimage would like to know the output file size.
Initially this is the same as the input file size, but it may be affected by
adding headers, etc.

Add this information to the image parameters.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>