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

Add Atomic<T> template ...

Add a C++ `Atomic<T>` template to make atomics even easier. Basically
compatible with C++11 `std::atomic<T>`, but using the underlying
`core_util_atomic_xxx` functions from mbed_atomic.h, so appropriate for
synchronising with interrupts and optimised for uniprocessor.

One extra piece of functionality beyond the `core_util_atomic_xxx`
functions is the ability to have an arbitrary atomic type - eg a small
structure with 2 `uint16_t`s can be stored in a `uint32_t` container.

Watchdog: remove VirtualWatchdog ...

We will provide documentation how to create your own VirtualWatchdog. It's simple,
create timeout and watchdog objects.

This class brought lot of discussion and questions. After our refactor we made this class
just a linked list of objects - something tickers can do as well (they already have linked list) and handling hw should be
done via Watchdog due to the limitations (timeout can be set only once per app!).

mbed: add Watchdog and VirtualWatchdog

Add HwWatchdog ...

- mbed_watchdog_mgr has interface name mbed_wdog_manager_start(),mbed_wdog_manager_stop(),mbed_wdog_manager_kick()
    - HwWatchdog is going to attach with LowPowerTIcker for periodic callback functionality
    - mbed_wdog_manager_start() will either get start either by BL/RTOS Aps,it reads the timeout value specified via macro and macro gets defined in target.json file.
    - mbed_wdog_manager_start() internally configure below HAL hw watchdog with timeout specified via target.json
    - mbed_wdog_manager_start() internally divide the timeout(specified in target.json) by the 2 and attach LowPowerTicker with periodic callback of hw_kick()
    - mbed_wdog_manager_start() internally create one instance of sw watchdog class,to access the static list data structure of sw watchdog class
    - mbed_wdog_manager_kick() function periodically get called and refresh the hw watchdog to avoid watchdog reset
    - converted C++ code into C based APIs
    - added boolean to control watchdog start and stop
    - Added detach from ticker on stop API

Add Watchdog and ResetReason headers to mbed.h ...

Adding headers to mbed.h will prevent you having to manually include
them when using the drivers in an application.

Assembler atomics ...

Reimplement atomic code in inline assembly. This can improve
optimisation, and avoids potential architectural problems with using
LDREX/STREX intrinsics.

API further extended:
* Bitwise operations (fetch_and/fetch_or/fetch_xor)
* fetch_add and fetch_sub (like incr/decr, but returning old value -
  aligning with C++11)
* compare_exchange_weak
* Explicit memory order specification
* Basic freestanding template overloads for C++

This gives our existing C implementation essentially all the functionality
needed by C++11.

An actual Atomic<T> template based upon these C functions could follow.

Rename lock functions and classes ...

Invert the name of the lock functions and classes so you are not
locking a negative.

Overhaul MPU for new requirements ...

Make the following changes:
-Allow a vector specific ARM MPU driver by defining MBED_MPU_CUSTOM
-Allow ROM address to be configured for ARMv7-M devices by
    setting the define MBED_MPU_ROM_END
-Add ROM write protection
    -Add new functions and lock
    -enable at boot
    -disable during flash programming

Rename MpuXnLock ...

Rename MpuXnLock  to ScopedMpuXnLock so it has the same naming
convention as ScopedMutexLock. Also make this class inherit from
NonCopyable to prevent misuse.

Add an RAII class for MPU execute never ...

Add the class MpuXnLock which uses RAII  to allow execution from RAM
in a block of code.

MBED_NO_GLOBAL_USING_DIRECTIVE is added to remove auto-addition of namespace ...

Macro guard `MBED_NO_GLOBAL_USING_DIRECTIVE` is added around namespace, to avoid
polluting users namespace.

Add QSPI header file in mbed.h

Separate version header file to get version updates in Mbed OS

Add stats header file to mbed.h ...

All API header files should be part of mbed.h

Merge pull request #6230 from bulislaw/system_reset ...

Add system_reset call

Merge pull request #5911 from deepikabhavnani/common_crc ...

CRC class implementation

Add system_reset() function to Mbed OS

Moved specialized functions to CPP file ...

Template specialized functions are moved to cpp file, to add MbedCRC.h
in mbed.h. Else linker shall multiple definition error.

Removed init/deinit and added cpp file for tables

platform: Add ScopedLock

platform: add DeepSleepLock ...

RAII object for disabling, then restoring the deep sleep mode

platform: add CriticalSectionLock ...

Critical section class RAII addition

FileSystem: Reintegrated FileSystemLike interface ...

Required for other representations of FileSystems, ie LocalFileSystem

Introduces FileSystemHandle for the same behaviour as FileHandle and
DirHandle.

Requires the following to hook into file/dir lookup:
```
int open(FileHandle **file, const char *filename, int flags)
int open(DirHandle **dir, const char *path)
```

This hook is provided by the FileSystem class, so requires no changes
from implementations.

Add APIs to namespace mbed ...

Certain public APIs are being added to mbed.h so as to make them
part of namespace mebd.

APIs being added are:

	* drivers/UARTSerial.h
	* platform/ATCmdParser.h
	* platform/mbed_poll.h
	* netsocket/nsapi_ppp.h

mbed: version macros are not defined on master ...

Remove mbed OS and mbed 2 version macros. As master can break anytime (should
not break but can happen and shall be expected that there are changes that might affect the bleeding edge developers), there is no remedy for protecting
the sw via macros (either using some special versions, etc). If it is not
defined, means it is developed version of mbed OS/mbed 2 and it is up to a user
to track changes.

Any release of mbed OS 5 and mbed 2 contains these version macros. Thus won't break anything.

Merge pull request #4185 from sarahmarshy/mbed-debug-include ...

Include mbed_debug.h in mbed.h

Set mbed minor and patch version to 0 for master ...

The proper version will be set for releases.

Include mbed_debug.h in mbed.h

Filesystem: Moved retarget related file interfaces into platform