From version 2021.09 U-Boot will fixup Turris Omnia's DTB before
booting, separating U-Boot's environment into separate MTD partition
"u-boot-env" [1].
Check if "u-boot-env" MTD partition exists and set the uci defaults
accordingly.
[1] https://lists.denx.de/pipermail/u-boot/2021-July/455017.html
Signed-off-by: Marek Behún <marek.behun@nic.cz>