build: Set ccache base to build top

This enables ccache sharing between different build tree paths.

Change-Id: I99636bdd2779a6ccf0649f027e35cf67df44b6ae
Don't set CCACHE_BASEDIR if its already set

 * enable use of shared ccache

Change-Id: Ic708a5d5169291d674b167aa2c87bf2c8adaafae
envsetup: export ANDROID_BUILD_TOP earlier

Change-Id: Ia3497f61f4e9aaeeb793cecd3b4f488f0a48181a
envsetup: Mark zsh as compatible

Basic functionality has been tested on zsh 5.0.2 and has been found to
work correctly.

Change-Id: I02e5e3bedf56b43104c280d9737ae7b334357643