configs/seli8: add u-boot update defconfig

configs/seli8: add u-boot update defconfig

This patch adds default defconfig for u-boot update version, the u-boot
update defconfig is a copy of the default (factory) defconfig with:
- adapted text base and environment addresses
- explicit flag that this is a field updated u-boot version

At the time of implementation this version is only used to verify the
update procedure, in future depend on the needs this defconfig can be
extended with additional options.

Signed-off-by: Aleksandar Gerasimovski <aleksandar.gerasimovski@hitachienergy.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>