tools: imx8m_image: fix warning message

When a firmware file is missing the warning message doesn't indicate the
firmware file name because '$tmp' var doesn't exist.
Fix the warning message and while at it reduce the if/else statement.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>

tools/imx8m_image.sh

@@ -14,10 +14,8 @@ for f in $blobs; do
 		continue
 	fi
 
-	if [ -f $f ]; then
-		continue
-	else
-		echo "WARNING '$tmp' not found, resulting binary is not-functional" >&2
+	if [ ! -f $f ]; then
+		echo "WARNING '$f' not found, resulting binary is not-functional" >&2
 		exit 1
 	fi
 done