Fix travis CI script bug

Signed-off-by: Huang Rui <vowstar@gmail.com>

.travis.yml

@@ -57,7 +57,7 @@ script:
   do
     echo "[MAKE]" $TRAVIS_BUILD_DIR/src/$DIR
     cd $TRAVIS_BUILD_DIR/src/$DIR
-    if [ ! -f "$Makefile" ]; then
+    if [ ! -f "Makefile" ]; then
       mkdir build && cd build
       cmake -DPROJ=$DIR -DTOOLCHAIN=$TRAVIS_BUILD_DIR/cache/kendryte-toolchain/bin ../../../ || exit -1
       make || exit -1