symbols for config_deviceHasVariableButtonBrightness

Summary:
Bring in framework later if we want. This helps m7 and s4 keep
device trees proper

Change-Id: Iafca8a2173513e2bd2c7ecba0b7e4e785e358d16

Test Plan: merge

Reviewers: synergy

Differential Revision: http://review.codefirex.com/D491