run-build.sh: Quote $CODE_COVERAGE_EN in case it's not defined

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I44fa742b414fb71846c53c6bda0fb312b366672e
diff --git a/run-build.sh b/run-build.sh
index c01c6b3..1c92425 100755
--- a/run-build.sh
+++ b/run-build.sh
@@ -67,7 +67,7 @@
     cmake_build_cmd=${cmake_build_cmd/-- install/-- $BUILD_TARGET}
 fi
 
-if [ $CODE_COVERAGE_EN = "TRUE" ] && [[ $CONFIG_NAME =~ "GCC" ]] ; then
+if [ "$CODE_COVERAGE_EN" = "TRUE" ] && [[ $CONFIG_NAME =~ "GCC" ]] ; then
     cmake_config_cmd=${cmake_config_cmd/toolchain_GNUARM.cmake/toolchain_GNUARM.cmake -DTFM_CODE_COVERAGE=True}
     echo "Flag: Add compiler flag for build with code coverage supported."
     echo $cmake_config_cmd