sched/fair: optimize idle cpu selection for boosted tasks

find_best_target CPU selection is biased towards lower CPU IDs. Bias
towards higher CPUs for boosted tasks. For boosted tasks unconditionally
use the idle CPU returned by find_best_target.

BUG: 29512132
Change-Id: I3d650051752163fcf3dc7909751d1fde3f9d17c0

Conflicts:
	kernel/sched/fair.c