imx: imx_cntr_image.sh: prevent warning for missing spl

when building U-Boot on imx8qxp and the board port uses
SPL, U-boot build shows

WARNING '.../spl/u-boot-spl.bin' not found, resulting binary is not-functional

This is because U-Boot binary is build first and Makefile
calls script imx_cntr_image.sh which checks if files
exists... but of course as spl is not yet build the
file `spl/u-boot-spl.bin` does not exist yet, so prevent
this warning.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/tools/imx_cntr_image.sh b/tools/imx_cntr_image.sh
index 972b95c..07acd38 100755
--- a/tools/imx_cntr_image.sh
+++ b/tools/imx_cntr_image.sh
@@ -14,6 +14,10 @@
 		continue
 	fi
 
+	if [ $f = "spl/u-boot-spl.bin" ]; then
+		continue
+	fi
+
 	if [ -f $f ]; then
 		continue
 	fi