Lots of usability improvements for the I2C API. Better docs and new top-level functions. (#64) ...

* Lots of usability improvements for the I2C API.  Better docs and new top-level functions.

* Document frequencies

* Tabs to spaces

* More style fixes

* Run astyle

* Clean up docs

* Add note about addressing, change 10 bit to 11 bit

* Fix spellcheck

* Fix paste error

* Oops, fix accidental change

Fix overflow issue causing common tickers test to intermittently fail (#57) ...

* Fix overflow issue causing common tickers test to intermittently fail (#44)

* Tabs -> spaces

Oops fix tabs

Disable mbed semihosting interface in tests, so as not to mess up their timing

Rename inconsistently named testing-related variables. Also fix STM32CUBE not working from application code.

Provide MBED_RAM_SIZE define allowing greentea tests to compile

Provide MBED_RAM_SIZE symbol for MUSCA_S1

Add more missed test skips

Fix some tests that weren't being skipped

Fix typo preventing unification test from running

Convert the rest of the hal tests, convert mbedtls to an OBJECT library

Fix a few more HAL tests

Enable reset_reason HAL test

- Split mbed-core and mbed-rtos into -sources and -flags targets ...

- Fix some logic errors in top level cmakelists related to building greentea tests
- Fix mbed_create_distro() for OBJECT libraries

Clean up top-level build file and build options, fix build issues on MinGW

fix: typo in pwmout_api.h (pulsewitdth -> pulsewidth)

GPIO: Use uintptr_t for gpio_irq_api context ...

The HAL gpio_irq_api stores object IDs, which serve as a form of context
for the dispatch of the interrupt handler in the drivers level
InterruptIn Class. The way this is achieved is that the InterruptIn
Class casts its address to uint32_t, which is stored as the ID.
This results in compilation failure when the size of an object pointer
is greater than uint32_t, for example when building on a PC for unit
testing.

In order to allow Unit Testing of the InterruptIn Class, we replace the
use of uint32_t with uintptr_t (type capable of holding a pointer),
which allows portability and expresses intentions more clearly.
In aid of this latter goal, we also replace the use of the name "id"
with "context", to improve clarity - these are addresses of the context
related to that callback.

pin_names-arduino_uno test : UART test case update

CAN: Use uintptr_t for can_irq_ids ...

The HAL can_api stores an array of IDs in order to dispatch interrupts
to the correct CAN object. The drivers level CAN Class casts a pointer
to itself to an uint32_t, which is stored as the ID and then cast back
to a CAN * in order to call the correct handler. This results in
compilation failure when the size of an object pointer is greater than
uint32_t, for example when building on a PC for unit testing.

In order to allow Unit Testing of the CAN Class, we replace the use of
uint32_t with uintptr_t (type capable of holding a pointer), which
allows portability and expresses intentions more clearly. In aid of this
latter goal, we also replace the use of the name "id" with "context",
to improve clarity. These are addresses of the context related to that
callback.

Merge pull request #15018 from jeromecoutant/PR_FLASHTEST ...

hal-tests-tests-mbed_hal-flash compilation warning

Merge pull request #15012 from felser/MTS_DRAGONFLY_L496VG ...

Add platform MTS_DRAGONFLY_L496VG

hal-tests-tests-mbed_hal-flash compilation warning

Add platform MTS_DRAGONFLY_L496VG

Unit tests: Add stubs for analogin_api.c functions ...

Currently there are no stub implementations of the analogin_api.c
functions. As the AnalogIn class makes use of these functions, these
stub definitions are required in order to build AnalogIn.cpp and
generate .gcno files to be used to generate code coverage metrics.

CMake: Add option to enable greentea tests ...

Add an option to enable the greentea tests independently from the unit
tests.

We can't just use the typical BUILD_TESTING option to enable greentea
tests. BUILD_TESTING enables unit tests and fetches googletest, which
are compiled for the host. Greentea tests are cross compiled and require
a toolchain file. For this reason we add a new option just to enable
greentea tests, preventing build failures triggered by the unit tests
and googletest.

Unit tests: Fix pin validation failure ...

The test script pinvalidate.py requires the following which are
missing in the unit test stub PinName.h:
* A comment "MBED TARGET LIST"
* `CONSOLE_TX` and `CONSOLE_RX` in the `PinName` enum

This commit adds them.

Unit tests: Remove redundant CMake target mbed-headers-base ...

Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Unit tests: Fix style in hal gpio_object.h stub

Unit tests: Move hal target_h stubs ...

Stubs previously in UNITTESTS/target_h/ have the same names as
regular Mbed OS headers, intending to override the latter directly.
We move hal target_h stubs into hal/tests/UNITTESTS/doubles/.

Note: In Mbed OS, the standard include format requires each header to
be prefixed with its module name, for example "hal/gpio_api.h". This
requires headers to be organized in a module directory. But unit tests
stubs for hal correspond to what a real Mbed target would have
implemented (in a non-test scenario), and targets do not currently put
headers in hal/, so we similary put stub headers directly in
hal/tests/UNITTESTS/doubles/ instead of add a hal/ subdirectory there.

pinvalidate.py script: add deprecated ARDUINO test