hwconfig: Allow to use restricted env

During early boot phase GD_FLG_ENV_READY is not set but env_get() may work
when env is ready in restricted mode. Do not fail with error message
"WARNING: Calling __hwconfig without a buffer and before environment is ready"
when env is already working by checking for ENV_VALID flag.

Signed-off-by: Pali Rohár <pali@kernel.org>