test/py/tests/test_fpga.py: Correct check for legacy image format support

The build option to support images of type 'IMAGE_FORMAT_LEGACY' is
CONFIG_LEGACY_IMAGE_FORMAT so update the pytest to check for the correct
option.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>