mips: mt76xx: gardena-smart-gateway: Enable CONFIG_USE_PREBOOT

Enable CONFIG_USE_PREBOOT on for the gardena mt7688 platforms, so that
this feature can be used here.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/configs/gardena-smart-gateway-mt7688_defconfig b/configs/gardena-smart-gateway-mt7688_defconfig
index 14c0254..ad2fbd0 100644
--- a/configs/gardena-smart-gateway-mt7688_defconfig
+++ b/configs/gardena-smart-gateway-mt7688_defconfig
@@ -15,6 +15,7 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="cp.b 83000000 84000000 10000 && dhcp uEnv.txt && env import -t ${fileaddr} ${filesize} && run do_u_boot_init; reset"
+CONFIG_USE_PREBOOT=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOARD_EARLY_INIT_F=y