Make sure os_get_current_time_us wraps as 32bit boundary

[elupus]

Extend the timer used to 64bit by keeping track of when it wraps

Background
When kernel is running at tick rate different from 1000 ticks per seconds:

• tick_to_ms will not wrap at 32 bit boundary
• tick_to_ms will degrade to 1 second accuracy when tick is larger than UINT32_MAX / 1000

This lead to issues when os_get_current_time_us is used as a periodic timing manager as in the following example:

uint32_t diff = os_get_current_time_us() - start;
if (diff > delay)
   do_something()

This is welled formed if os_get_current_time_us() always wraps around at 32bit max boundary, but lead to unexpected behaviour otherwise.

[elupus]

FreeRTOS port suffers equal problems.