env: Setup default value for ENV_OFFSET_REDUND

env: Setup default value for ENV_OFFSET_REDUND

This variable is used for pointing to location where redundant variables
are located. There is no default value. And it doesn't need to be specified
which is showing as warning when savedefconfig is called.

make xilinx_zynqmp_virt_defconfig
 #
 # configuration written to .config
 #
make savedefconfig
 scripts/kconfig/conf  --savedefconfig=defconfig Kconfig
 .config:199:warning: symbol value '' invalid for ENV_OFFSET_REDUND

Signed-off-by: Michal Simek <michal.simek@xilinx.com>