tools: ensure zeroed padding in external FIT images

Padding the header of an external FIT image is achieved by truncating
the existing temporary FIT file to match the required alignment before
appending image data. Reusing an existing file this way means that the
padding will likely contain a portion of the original data not
overwritten by the new header.

Zero out any data past the end of the new header, and stop at either
the end of the desired padding, or the end of the old FIT file,
whichever comes first.

Fixes: 7946a814 ("Revert "mkimage: fit: Do not tail-pad fitImage with external data"")
Signed-off-by: Roman Azarenko <roman.azarenko@iopsys.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>