Merge "build: Set ccache base to build top" into cm-11.0
(cherry picked from commit 14e552ee7b538f782edbb9aa0e267d671d86e3cf)
Don't set CCACHE_BASEDIR if its already set

 * enable use of shared ccache

Change-Id: Ic708a5d5169291d674b167aa2c87bf2c8adaafae
(cherry picked from commit 3bb6b8b4af13ee45ff0f4a1246126f4fc549d2bc)