When core 1 repeatedly calls epic_sleep(), this could delay an
epic_load() from core 0 because the reset interrupt can only be
delivered after such a call completed.
To work around this, add calls into do_load() to notify the dispatcher
task that it should return early from calls like epic_sleep(). This
gets a bit tricky because the dispatcher task also uses
ulTaskNotifyTake() to wake up when new API calls arrive. To make sure
we still break out of epic_sleep(), we need to call
xTaskNotifyGive() twice in the wait loop.