mcs: Add a scheduling context object

This is the first part of the seL4 MCS. This commit:

    * adds a scheduling context object. Threads without scheduling
      context objects cannot be scheduled.
    * replaces tcbTimeSlice with the scheduling context object
    * adds seL4_SchedControl caps for each core
    * adds seL4_SchedControl_Configure which allows users to configure
      amount of ticks a scheduling context has, and set a core for the
      scheduling context.
    * adds seL4_SchedContext_Bind, Unbind and UnbindObject, which allows
      a tcb to be bound to a scheduling context.