board: tbs2910: Disable CONFIG_ENV_VARS_UBOOT_CONFIG in defconfig

This is not required for sysboot (we defined fdtfile), let's save a few
bytes in the binary image without these variables.

Signed-off-by: Soeren Moch <smoch@web.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>