OpenPicoRTOS v1.9.2

[jnaulet]

The main new feature in this release is the management of the CPU clock frequency inside the
ports, which allow new API functions like arch_delay_us with decent accuracy.

Clock drivers are responsible for setting this correctly & have been updated.
From the user's perspective it's one less parameter to worry about
(except for the AVR architecture).

Tasks statistics have been improved without adding too much overhead and in was a real PITA !
I hope it's the final version.