mkimage: do not fail if there is no print_header function

mkimage: do not fail if there is no print_header function

Commit 253c60a 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 <[email protected]>
Cc: Philipp Tomsich <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Tom Rini <[email protected]>
Reviewed-by: Philipp Tomsich <[email protected]>