[meta-xilinx] uboot-utils fw_setenv ends up with wrong CRC in environment

[star at gmx.li]

With update to yocto2.3 I started using the vanilla 4.9 kernel. (currently out of tree, as I don't know howto build a recipe to build from kernel.org sources). 
This works for me with minor changes (backporting the spi-zynq-qspi.c).

But I found a strange problem. The tool "fw_setenv" (out of u-boot utils) doesn't work anymore - after usage I get a destroyed environment. ("fw_readenv works - but only until usage of fw_setenv).
I checked a few things, but I end up, that just changing the kernel is the issue (neither u-boot, nor the utils, nor the devicetree). I am wonding what can be the reason for that? Is some kernel setting missing?