x86: zimage: Quieten down the zimage boot process

Much of the output is not very useful. The bootm command is quite a bit
quieter. Convert some output to use log_debug().

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