Export test - multiple failures

[0xc0170]

=== Exporting to "gcc_arm::LPC4088_DM" ===
Traceback (most recent call last):
  File "tools\export_test.py", line 337, in <module>
    test_export(toolchain, target)
  File "tools\export_test.py", line 59, in test_export
    zip_path, report = export(USER_PRJ, USR_PRJ_NAME, toolchain, target, base_dir, temp_dir, False, None, fake_build_url_resolver)
  File "\tools\export\__init__.py", line 106, in export
    exporter.generate()
  File "\tools\export\gccarm.py", line 170, in generate
    ctx[key] = ctx['vpath'][0] + "/" + ctx[key]
TypeError: cannot concatenate 'str' and 'NoneType' objects

This error is shown for some targets, for uvision:

=== Exporting to "uvision::DISCO_F303VC" ===
Traceback (most recent call last):
  File "tools\export_test.py", line 337, in <module>
    test_export(toolchain, target)
  File "tools\export_test.py", line 59, in test_export
    zip_path, report = export(USER_PRJ, USR_PRJ_NAME, toolchain, target, base_dir, temp_dir, False, None, fake_build_url_resolver)
  File "C:\Code\git_repo\github\mbed-official\tools\export\__init__.py", line 95, in export
    if not ProGenDef(ide).is_supported(TARGET_MAP[target].progen['target']):
  File "C:\Code\git_repo\github\mbed-official\tools\targets.py", line 169, in __getattr__
    v = self.__getattr_helper(attrname)
  File "C:\Code\git_repo\github\mbed-official\tools\targets.py", line 161, in __getattr_helper
    raise AttributeError("Attribute '%s' not found in target '%s'" % (attrname, self.name))
AttributeError: Attribute 'progen' not found in target 'DISCO_F303VC'

cc @theotherjimmy @sarahmarshy

[ciarmcom]

ARM Internal Ref: IOTMORF-132

[sg-]

Closing all older exporter related issues. Please reopen if still valid. https://developer.mbed.org/blog/entry/Fixed-exporters-in-the-mbed-Online-Compi/