configs/ci20: update kernel and u-boot

Unbreak failing defconfig.
Resulting kernel and rootfs booted on ci20 device.

While at it, drop the useless BR2_TARGET_UBOOT_VERSION variable, this
variable is a hidden variable, so it is useless to mention it in a
defconfig.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas: drop BR2_TARGET_UBOOT_VERSION.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>