build: hook the schedulers into Kconfig

build: hook the schedulers into Kconfig

Allow the schedulers to be independently enabled or disabled at
compile-time. To match existing behavior, all four schedulers are
compiled in by default, although the Credit2, RTDS, and ARINC653 are
marked EXPERIMENTAL to match their not currently supported status.

Signed-off-by: Jonathan Creekmore <jonathan.creekmore@gmail.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Dario Faggioli <dario.faggioli@citrix.com>