configs/armadeus_apf27: fix U-Boot configuration

The U-Boot part of the defconfig was not specifying explicitly any
U-Boot version. Since commit 21e3ae8a
("boot/uboot: default to kconfig buildsystem for latest version"), we
default to using the kconfig build system when the default U-Boot
version is used. Following this change, the apf27 defconfig therefore
started using kconfig, for which the BR2_TARGET_UBOOT_BOARDNAME
Config.in option is not used. Due to this, the build fails with:

boot/uboot/uboot.mk:411: *** No board defconfig name specified, check your BR2_TARGET_UBOOT_BOARD_DEFCONFIG setting.  Stop.

Indeed, when Kconfig is used, the board defconfig must be specified
with BR2_TARGET_UBOOT_BOARD_DEFCONFIG.

As part of fixing this, we also set a fixed U-Boot version for this
defconfig, like we do in all other defconfigs.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/123771003



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>