omap3: overo: Fix fdtfile test

Commit 12cc54376768461533b55ada1b0b6d4979f40579 'omap3: overo: Select
fdtfile for expansion board' wrongly missed the operator in the fdtfile
test. Update the test to only overwrite an empty fdtfile environment
variable.

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
1 file changed