[Exporters] GCC_ARM export is broken

[screamerbg]

$ mbed export -i gcc_arm -m K64F -n test-my-thing
Traceback (most recent call last):
  File "/private/tmp/mbed-os/core/tools/project.py", line 213, in <module>
    tmp_path, report = export(project_dir, project_name, ide, mcu, project_dir[0], project_temp, clean=clean, make_zip=zip, extra_symbols=lib_symbols, sources_relative=sources_relative)
  File "/private/tmp/mbed-os/core/tools/export/__init__.py", line 106, in export
    exporter.generate()
  File "/private/tmp/mbed-os/core/tools/export/gccarm.py", line 134, in generate
    self.resources.relative_to(curdir)
  File "/private/tmp/mbed-os/core/tools/toolchains/__init__.py", line 150, in relative_to
    self.features = {k: f.relative_to(base, dot) for k, f in self.features.iteritems()}
  File "/private/tmp/mbed-os/core/tools/toolchains/__init__.py", line 150, in <dictcomp>
    self.features = {k: f.relative_to(base, dot) for k, f in self.features.iteritems()}
AttributeError: 'NoneType' object has no attribute 'relative_to'
[mbed] ERROR: "python" returned error code 1.
[mbed] ERROR: Command "python -u /private/tmp/mbed-os/core/tools/project.py -i gcc_arm -m K64F -n test-my-thing --source ." in "/private/tmp/mbed-os"

cc @0xc0170