Linker set based initialization

[Sebastian Huber]

This patch set implements fine grained linker set based initialization.  The
basic framework is already in place for several weeks.  See ticket
https://devel.rtems.org/ticket/2408 for the details.  The individual patches
contain mostly mechanical changes.  There are virtually no changes in the test
suite (except sptests/spsysinit01 and fstests/fsnofs01).  We are one big step
further to always enable the POSIX API.