Performance profiles

This changes adds support for custom performance profiles through overlays. Supported
devices should define config_perf_profile_prop with the proper system property to be set
and fill an init.rc file with the appropiate handlers for every defined performance profile.

Patchset 2: Rebased
            New simplified strings
Patchset 3: Fix build
Patchset 4: Rebased

Change-Id: I5a6612db94da3c09d8ec2e8920bf9516f62d0024
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>