sched: trigger immediate migration of tasks upon boost

Currently turning on boost does not immediately trigger migration of
tasks from lower capacity cpus. Tasks could incur migration latency
of up to one timer tick (when check_for_migration() is run).

Fix this by triggering a migration check on cpus with lower capacity
as soon as boost is turned on for first time.

Change-Id: I244649f9cb6608862d87631325967b887b7f4b7e
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>