Turned ENABLE_CPUSETS into runtime decision.

ENABLE_CPUSETS is usually set in BoardConfig.mk to use CONFIG_CPUSETS
feature of Linux kernel.

This change turns the build time variable into runtime decision by
checking if a special file, "tasks", can be found under the mount
point, /dev/cpuset, for cpusets.

Bug: 34726944

Test: Tested on Sailfish
          without cpuset mounted
	  with cpuset mounted, but without schedtune mounted
	  with cpuset mounted and with schedtune mounted

Change-Id: I1c044c3c9d4d15360a1605e7022da84d94df4014