mkimage: do not fail if there is no print_header function

Commit 253c60a5 breaks the exit value of 'mkimage -T rkimage'
and print the following  error:
  mkimage: Can't print header for Rockchip Boot Image support: Success

It is not a failure to not print headers, so just display the warning message,
and finish the function properly.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>

Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>