mkimage: Fix variable length header support

mkimage: Fix variable length header support

Support for variable length images like AIS image was introduced
in commit f0662105b674a3874227316abf8536bebc9b5995. A parameter
"-s" was also introduced to prohibit copying of the image file
automatically in the main program. However, this parameter
was implemented incorrectly and the image file was copied
nevertheless.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>