Updated event period handling and greentea test ...

non_periodic constant has been moved out of the Event class and
made static within the events namespace so that it is available
both internally within the class and externally.

The MBED_ASSERT has been changed to MBED_WARNING.

The greentea test has been updated:
1) timings reduced to make the test cases run faster
2) The call handler simplified

Update events period method to check for invalid values ...

The period method currently allows any ms value positive, negative
and zero. A negative value means dispatch a non periodic event
ie just once. 0 is unspecified behaviour. This commit forces the
user to use either positive periods or a new constant non_periodic
and should any other value be provided will default to
non_periodic.

A Greentea test case is also provided to check this works as
expected.

CMake: Add CMake support for events greentea test

Add SPDX license identifier to Arm files ...

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

Remove deprecated local unit tests

CI & local unit test fixes

Restructure events directory & move tests