RTOS API for bare metal ...

Provide partial RTOS API for bare metal builds - things that
can be done in a single threaded environment.

Allows more code to work in both RTOS and bare metal builds without
change, and in particular gives easy access to the ability to
efficiently wait for something occurring in interrupt.

Available in bare-metal:
* ThisThread
* osThreadFlagsSet to set flags on main thread (can be set from IRQ)
* EventFlags (can be set from IRQ)
* Semaphores (can be released from IRQ)
* Mutex (dummy implementation)

Not useful:
* ConditionVariable (could only be signalled from 2nd thread)
* RtosTimer (calls in a second thread context)
* Thread

Unimplemented:
* Mail, Queue, MemoryPool

Possible future work:
* ConditionVariableCS to act as IRQ signalled ConditionVariable

Introduce Semaphore::acquire methods ...

Deprecate wait() in favour of acquire(), try_acquire(),
try_acquire_for() and try_acquire_until().

Brings Semaphore more into line with CMSIS-RTOS 2 (which uses "acquire"),
itself (as it has "release"), and other classes having "try", "try for"
and "try until".

Also steps away from vague "wait" term - the primary operation here is
to acquire the semaphore, and this will of course sleep.

Clean up Thread deprecation warnings ...

Static Thread methods and signal methods have been deprecated. Remove
all references in the main code, and most of the tests. Some tests of
the deprecated APIs themselves remain.

Add ConditionVariable::wait_until ...

Given the 64-bit timebase, add wait_until to ConditionVariable.

Move the timeout example to wait_until(), and give wait_for() an
alternative example, as it's no longer the best option for a
timeout.

Tidy up - remove the redundant RESUME_SIGNAL definition.

Fix and add test for ConditionVariable ...

Fix the circular linked list handling in ConditionVariable and add a
test to validate the linked list implementation.

Add ConditionVariable to mbed rtos ...

Add the ConditionVariable class to mbed rtos to provide easier and
safer synchronization.