Convert CONFIG_CMD_ZBOOT to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ZBOOT

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>
diff --git a/arch/Kconfig b/arch/Kconfig
index 08f6e7f..78532f5 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -113,6 +113,7 @@
 	imply CMD_IO
 	imply CMD_IRQ
 	imply CMD_SF_TEST
+	imply CMD_ZBOOT
 
 config XTENSA
 	bool "Xtensa architecture"