env_nand_init() says the environment is valid even if it is assigning
default environment due to not being able to access nand pre-reloaction
(determined by macro values). Change this to ENV_INVALID and let the
generic env_init() function, which calls this initializer, assign the
default environment.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>