build: Set ccache base to build top

This enables ccache sharing between different build tree paths.

Change-Id: I99636bdd2779a6ccf0649f027e35cf67df44b6ae
build: no longer build emulator by default

This is just increasing build times when most people are not using it.
Let's speed up builds a bit.

Building emulator should still be possible by appending BUILD_EMULATOR=true
to the build command

PS2: Let's keep the darwin warning.

Change-Id: Ibd8e2b90455bd603a691c7b95466068b6fae212a
Signed-off-by: Andrew Warfield <hiasant@gmail.com>