[timers] add platform.get_time_from_boot() which will eventually replace platform.get_ticks_from_boot()
[timers] replace arm cortex m get_ticks_from_boot() implementation with get_time_from_boot() internals
[timers] replace direct calls to cortex_m_get_ticks_from_boot() with platform_get_ticks_from_boot(). This would allow overriding the default behaviour
[timers] support get_time_from_boot() on unix
[timers] replace platform.get_ticks_from_boot implementations with platform.get_time_from_boot(). get_ticks_from_boot is no longer platform API. Higher layers still use it though.