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

Edit Kernel.h ...

Make minor copy edits for active voice, branding and deletion of extra spaces.

Updated attach_thread_terminate_hook doxygen comments

Kernel.h doxygen update

Add ThisThread

Add Kernel::get_ms_count ...

Give C++ access to the RTOS's absolute timebase, reducing the need to
run private Timers and similar. Allows wait_until functionality, and
makes it easier to avoid time drift.

Place it in a new header and namespace in case we want more kernel
functions in future.

Try to cover over the breaking API change potentially upcoming in
CMSIS-RTOS 2.1.1, when it reduces the tick count from 64-bit to 32-bit.
(See https://github.com/ARM-software/CMSIS_5/issues/277)

Explicitly state that ticks are milliseconds in mbed OS, despite CMSIS
RTOS 2 permitting different tick rates.

See also https://github.com/ARMmbed/mbed-os/pull/3648 (wait_until
for condition variables) and
https://github.com/ARMmbed/mbed-os/issues/5378 (EventQueue should
use RTOS tick count).