tools: mkimage : bugfix returns correct value for list command

List command always return "EXIT_SUCCESS" even in case of
failure by any means.

This patch return 0 if list command is sucessful,
returns negative value reported by check_header functions

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>