Clock functions

[larsbrinkhoff]

Start defining a separate unit which controls the system clock. This unit makes calls to other devices to advance their state machines.

• Separate clock.c file to make all clock callbacks.
• Added delay API to call a function in the future.

[larsbrinkhoff]

Oops, have to fix a bug.

[larsbrinkhoff]

Fixed. Go ahead.

[larsbrinkhoff]

The delay API keeps a linked list, sorted by time, with the time delta in each element. The elements are allocated with malloc. The API is used very frequently, e.g. every four clock cycle in the MMU, so this may impose an overhead. Maybe the element should be statically allocated in an array.