tools: sched: add support for 'null' scheduler

tools: sched: add support for 'null' scheduler

It being very very basic, also means this scheduler does
not need much support at the tools level (for now).

Basically, just the definition of the symbol of the
scheduler itself and a couple of stubs.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>