mkimage: ublimage must return if the header is not verified

Each image handler must return a not-zero velue if the
header is not recognized to allow the main program to
iterate to the next handler.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>