Convert CONFIG_CMD_SAVES to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SAVES

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/configs/ethernut5_defconfig b/configs/ethernut5_defconfig
index 26df916..ca82336 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y
 # CONFIG_CMD_FPGA is not set
+CONFIG_CMD_SAVES=y
 CONFIG_CMD_RARP=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y