* Initial attempt at rethinking the STM32 I2C v2 HAL.  Makes single-byte work properly and adds a new 'state' variable to track what the hardware is doing.

* Fix some initial test failures

* Fix incorrect logic

* Fix more incorrect logic

* Tabs to spaces

* Fix repeated starts with single-byte API

* Fix race condition causing stop() after nacked address to sometimes break things

* Fix missed i2c structs that should have been removed

* Fix doing a repeated start from single-byte to transaction API causing I2C peripheral to lock up

* Fix xferOperation being set wrong for repeated starts, causing the peripheral to hang

* Fix race condition with repeated start after single-byte operation

* Fix compilation for targets that use I2C IP v1

* Fix initialization of XferOperation for API v1, optimize stop()

* Remove unneeded line

* Add docs for I2C events

The TEST_SKIP and TEST_SKIP_UNLESS macros (w/ and w/o messages) allow the test
to skip the test case execution from the point the macro was called (without
failing the test).

This commit adds the following test frameworks:

- `greentea-client` (https://github.com/ARMmbed/greentea-client)
  - This framework provides a key-value api for communicating with the
    greentea test tool (https://github.com/ARMmbed/greentea)
- `unity` (https://github.com/ARMmbed/unity)
  - This framework provides test assert macros that can be used when
    writing test cases
- `utest` (https://github.com/ARMmbed/utest)
  - This framework allows you to execute a series of test cases with
    reporting that works with the greentea test tool
    (https://github.com/ARMmbed/greentea)

The following changes were made when bringing these frameworks into the
tree:

- References to `mbed_drivers/mbed.h` within utest's tests were migrated
  to `mbed.h`
- The yotta file `module.json` was removed from `greentea-client` and
  `unity`
  - `coverage.json` was also removed from `greentea-client`
- `.gitignore` and `.gitattributes` were removed from `greentea-client`
- Apache 2.0 license files were removed from `greentea-client`

This also brings in a number of tests that have been newly written or ported from various sources:

- `TESTS/integration` - Very basic tests, used to check if testing frameworks are working correctly
- `TESTS/mbed_drivers` (Thanks @PrzemekWirkus!) - TESTS ported from mbed OS 3.0 (https://github.com/ARMmbed/mbed-drivers)
- `TESTS/mbedmicro-mbed` (Thanks @PrzemekWirkus!) - Tests that weren't covered by `TESTS/mbed_drivers` that currently live in `libraries/tests/mbed`
- `TESTS/mbedmicro-rtos-mbed` (Thanks @PrzemekWirkus!) - Ported tests that currently live in `libraries/tests/rtos/mbed`
- `TESTS/storage_abstraction` (Thanks @rgrover!) - Tests for the storage_abstraction hal