x86: zimage: Improve command-line debug handling

At present if the command line is very long it is truncated by the
printf() statement, which works within a limited buffer. Use puts()
instead. Also show better debugging with the command-line setup
fails.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>