configs/stm32f469_disco_sd_defconfig: explicitly set U-Boot version

The version of U-Boot must be explicitly set because if you keep the
default setting, the bootloader is continuously updated in buildroot with
the risk of building a non-working image.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit 924b163b1e4e1f82ea758a7e69a73f9e6b888e62)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/stm32f469_disco_sd_defconfig

@@ -18,6 +18,9 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_SIZE="32M"
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_TARGET_UBOOT=y
+BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2021.10"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="stm32f469-discovery"
 BR2_TARGET_UBOOT_NEEDS_OPENSSL=y
 BR2_PACKAGE_HOST_DOSFSTOOLS=y