Convert CONFIG_CMD_SH_ZIMAGEBOOT to Kconfig

This converts the following to Kconfig:
CONFIG_CMD_SH_ZIMAGEBOOT

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>