Fix copying CI targets

CI currently expects target tree inside targets directory and uses
a hardcoded path. The targets directory might not exist on some of
the repos tested, so create it first here. CI will be updated to use
the repo slug to determine which repo's targets it should build.

.travis.yml

@@ -116,6 +116,7 @@ before_script:
   - gcc --version
   - if [ "${TEST}" != "TEST_ALL" ]; then arm-none-eabi-gcc --version; fi
   - cp -R $HOME/ci/mynewt-nimble-project.yml project.yml
+  - mkdir -p targets
   - cp -R $HOME/ci/mynewt-nimble-targets targets
   - $HOME/ci/prepare_test.sh $VM_AMOUNT
   - mkdir -p repos && pushd repos/