score: Statically initialize IO manager

This simplifies the RTEMS initialization and helps to avoid a memory
overhead.  The workspace demands of the IO manager were not included in
the <rtems/confdefs.h> workspace size estimate.  This is also fixed as a
side-effect.

Update documentation and move "Specifying Application Defined Device
Driver Table" to the section end.  This sub-section is not that
important for the user.  Mentioning this at the beginning may lead to
confusion.