arm: mvebu: clearfog_defconfig: enable setexpr command

This command is useful in U-boot scripts and it is being used by
OpenWrt bootscript for this board [1]. Otherwise shell scripting
commands are enabled by default in cmd/Kconfig.

[1] https://github.com/openwrt/openwrt/blob/852126680e21edc71c0c66561ae5a6d7479dcc67/target/linux/mvebu/image/clearfog.bootscript#L7

[2] https://source.denx.de/u-boot/u-boot/-/blob/e95afa56753cebcd20a5114b6d121f281b789006/cmd/Kconfig#L1504



Fixes: 0299c90f ("arm: mvebu: Add
SolidRun ClearFog Armada 38x initial support")

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>