omap3: overo: Fix fdtfile test

Commit 12cc543767 '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>

include/configs/omap3_overo.h

@@ -151,7 +151,7 @@
 			"run mmcboot;" \
 		"fi;" \
 		"if run loadzimage; then " \
-			"if test $fdtfile; then " \
+			"if test -z \"${fdtfile}\"; then " \
 				"setenv fdtfile omap3-${boardname}-${expansionname}.dtb;" \
 			"fi;" \
 			"if run loadfdt; then " \