This way, the redirected env sessions for the CPU burner are no longer
called directly by init. Init stays independent from the behavior of the
CPU balancer.
Issue #3837
Issue #4029
* Remove SPEC declarations from mk/spec
* Remove all board-specific REQUIRE declaratiions left
* Replace [have_spec <board>] run-script declarations with have_board where necessary
* Remove addition of BOARD variable to SPECS in toplevel Makefile
* Move board-specific directories in base-hw out of specs