Convert CONFIG_CMD_EECONFIG to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_EECONFIG

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/gateworks/gw_ventana/Kconfig b/board/gateworks/gw_ventana/Kconfig
index ccce98e..c577984 100644
--- a/board/gateworks/gw_ventana/Kconfig
+++ b/board/gateworks/gw_ventana/Kconfig
@@ -9,4 +9,9 @@
 config SYS_CONFIG_NAME
 	default "gw_ventana"
 
+config CMD_EECONFIG
+	bool "Enable the 'econfig' command"
+	help
+	  Provides access to EEPROM configuration on Gateworks Ventana
+
 endif