sandbox: add test of CONFIG_ENV_IMPORT_FDT

Check that a variable defined in /config/environment is found in the
run-time environment, and that clearing fdt_env_path from within that
node works.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
[trini: Conditionalize the test being linked in]
Signed-off-by: Tom Rini <trini@konsulko.com>