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.

RTOS API: minor tidies ...

Remove unneeded memsets and an unneeded cast.

Add missing initialisation of EventFlags attributes - no all fields were
being initialised.

rtos: fix astyle

remove unused variable / resolve compiler warning

Add Semaphore::wait_until ...

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

Naming is based on Thread::wait_until.

pthreads uses "timedwait", but that's not a good fit against our
existing wait() - pthreads only has an absolute-time wait, not relative.

Shrink RTOS classes ...

Various RTOS classes were storing their CMSIS-RTOS creation attribute
structure as a member, when it's not required after construction. Reduce
memory by eliminating this member.

rtos: Unbreak semaphore, trade assert for saturation with original limit ...

Before rtx 5, the max count on semaphores was UINT16_MAX, aftewards it
was decreased to 1024 with an assert on overflow.

This is especially problematic for semaphores used for signaling, since
there is no replacement currently available in C++.

Update codebase for CMSIS5/RTX5 ...

Update all of mbed-os to use RTX5.

For drivers, events, hal, platform, rtos and mbed.h add one level of path to make sure specific and unique includes files are found.

restructure - Restructured rtos directory