Convert CONFIG_CMD_ETHSW to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ETHSW

Also enable it for sandbox to increase build coverage.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index ab09a9f..cc959f6 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -711,6 +711,14 @@
 	help
 	  Acquire a network IP address using the link-local protocol
 
+config CMD_ETHSW
+	bool "ethsw"
+	help
+	  Allow control of L2 Ethernet switch commands. These are supported
+	  by the vsc9953 Ethernet driver at present. Sub-commands allow
+	  operations such as enabling / disabling a port and
+	  viewing/maintaining the filtering database (FDB)
+
 endmenu
 
 menu "Misc commands"