Fix a shell issue of empty string
If the $(UBUNTU_PLATFORM) is an empty string, the test command will fail,
double quotes it to fix.
```
mock for platform
/bin/sh: line 0: [: =: unary operator expected
```
@@ -148,7 +148,7 @@ pylint:
$(MOCK_BIN):
@echo "--- mock for platform $(UBUNTU_PLATFORM)"
- @if [ $(UBUNTU_PLATFORM) = "Ubuntu" ]; then\
+ @if [ "$(UBUNTU_PLATFORM)" = "Ubuntu" ]; then\
pip install mock;\
else\
mkdir -p $(PYTHONSRC_INSTALL_SITE) && \
