env: Load env when ENV_IS_NOWHERE is only location selected

This patch prevent u-boot from hanging on a UltraZed EG board (zynqmp).

Without the patch,
(drv = env_driver_lookup(ENVOP_INIT, prio))
evaluates to 0, causing prio = 0
Then, (!prio) is hit, returning -ENODEV causing a stall.

With the patch,
instead of returning -ENODEV and causing a stall, we
set gd->env_addr (is this really needed?)
and then
mark gd->env_valid = ENV_INVALID to use the default env.