tools: improve portability of imx_cntr_image.sh

Replace non-portable operator == with =

The operator == in sh(1) / test(1) is non-POSIX and only implemented by
some shells (like bash). It is equivalent to the standard defined operator =.

tools/imx_cntr_image.sh

@@ -10,7 +10,7 @@ file=$1
 blobs=`awk '/^APPEND/ {print $2} /^IMAGE/ || /^DATA/ {print $3}' $file`
 for f in $blobs; do
 	tmp=$srctree/$f
-	if [ $f == "u-boot-dtb.bin" ]; then
+	if [ $f = "u-boot-dtb.bin" ]; then
 		continue
 	fi