Import Mbed 2 tools only if mbed.bld is present

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Additionally this resolves the cases where someone is using an empty SCM which doesn't have neither Mbed OS 5 nor Mbed 2 library references.
It also improves py.test speed.

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CI is failing because of pytest install failures (?!)

sudo pip install pytest

Downloading/unpacking pytest
  Downloading pytest-3.4.2-py2.py3-none-any.whl (189kB): 189kB downloaded
Downloading/unpacking pluggy>=0.5,<0.7 (from pytest)
  Downloading pluggy-0.6.0.tar.gz
  Running setup.py (path:/tmp/pip_build_root/pluggy/setup.py) egg_info for package pluggy
    /usr/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'python_requires'
      warnings.warn(msg)
    
    warning: no files found matching 'CHANGELOG'
    warning: no previously-included files matching '*.pyc' found under directory '*'
    warning: no previously-included files matching '*.pyo' found under directory '*'
Downloading/unpacking attrs>=17.2.0 (from pytest)
  Downloading attrs-17.4.0-py2.py3-none-any.whl
Downloading/unpacking funcsigs (from pytest)
  Could not find any downloads that satisfy the requirement funcsigs (from pytest)
  Some externally hosted files were ignored (use --allow-external funcsigs to allow).
Cleaning up...
No distributions at all found for funcsigs (from pytest)
Storing debug log for failure in /home/ubuntu/.pip/pip.log

sudo pip install pytest returned exit code 1