About clocks

[jnaulet]

As seen in issue #52, clocks are not handled very well until the scheduler is running.

It might be useful to manage the clocks from the OS (at least partially) and not from the drivers.

This might allow some flexibility, like getting rid of the cpu frequency config.

This is a double edged sword, though, as it will probably make debugging the clock drivers harder since the input frequency will always be right.

But it will allow a arch_delay() that will be useful to manage timeouts during the init phase