Convert CONFIG_CMD_SPL to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SPL

Note that trats does not actually use SPL, so this option can no-longer be
set.

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/cmd/Kconfig b/cmd/Kconfig
index 5ac8389..1ca91fc 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -276,6 +276,15 @@
 	help
 	  Poweroff/Shutdown the system
 
+config CMD_SPL
+	bool "spl export - Export boot information for Falcon boot"
+	depends on SPL
+	help
+	  Falcon mode allows booting directly from SPL into an Operating
+	  System such as Linux, thus skipping U-Boot proper. See
+	  doc/README.falcon for full information about how to use this
+	  command.
+
 endmenu
 
 menu "Environment commands"