Point OPROFILE_EVENTS_DIR to oprofile dir in source tree.

Otherwise oprofile will punt with the following error message when
processing results collected by perf events since the event descriptions
are not stored in the default location on the host.

profile: could not open unit mask description file
/usr/local/share/oprofile//arm/armv7/unit_masks

Change-Id: I4ff2f2a87546ed3606490596285fb3e466e8d661