xilinx: Return ENVL_NOWHERE instead of ENVL_UNKNOWN

The system refuses to boot without any environment, so return ENVL_NOWHERE when
there's nowhere to store the environment instead of ENVL_UNKNOWN.

This fixes that the board won't boot from eMMC when CONFIG_ENV_IS_IN_FAT is not
defined, for example. Similar for other combinations.

Fixes: 1025bd09 "xilinx: zynqmp: Add support for saving variables"

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>