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/xtfpga_defconfig b/configs/xtfpga_defconfig
index e52e62b..f28c05f 100644
--- a/configs/xtfpga_defconfig
+++ b/configs/xtfpga_defconfig
@@ -10,6 +10,7 @@
 CONFIG_AUTOBOOT_STOP_STR=" "
 CONFIG_CMD_ASKENV=y
 CONFIG_CRC32_VERIFY=y
+CONFIG_CMD_SAVES=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_DIAG=y