Removed build time variable ENABLE_CPUSETS.

Use runtime check, cpuset_enabled(), instead of build time varianble
usually set in BoardConfog.mk.

Bug: 34726944

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

Change-Id: I00153628db62abfbaa0ba75d571ba1bda0611ff3