Changes

testenv: osmo-dev: add hint for gen_makefile fail

When gen_makefile.py from osmo-dev fails, it is likely that the osmo-dev
git clone is outdated, for example if a new file with configure options
was added to osmo-dev.git and is now being used by testenv. Display a
hint for pulling this repository to the user.

Change-Id: Ia9ed23fa910876252093054f5f389865632c7593