driver list: implement iteration over linker section content

driver list: implement iteration over linker section content

Extend the drivers.c source file, iterate over the driver descriptors
pointer array which was collected in the linker list. This version only
requires the start symbol, because the list is NULL terminated (by means
of linker script instructions).

This implementation provides the same two "entry points" of iteration
(listing) and execution of the available drivers as an earlier version
did. Yet happens to internally execute both alternatives of traversing
the linked list that was created at runtime, as well as the array which
was constructed at link time. So that users can compare screen output
between implementation variants.