[built] Refactor makefiles to imporve unit tests -

* Add avrtest support
* Provide `XI_UNIT_TEST_TARGET` which can be set to:
    - `native`
    - `avr`
* Define most rules on the top level (`Makefile.rules`)
* External dependencies (e.g. _tinytest_ & _avrtest_) now
  reside under `src/import/` and should be checked-in

TODO: Document the build system in attempt to freeze it
[ci] Update Travis script to use new system