make it dirty
build: use the system's ccache by default

ccache 3.x supports direct mode (http://ccache.samba.org/manual.html#_the_direct_mode)
This is not supported in the prebuilt version. Direct mode has been shown to decrease build time by up to 40x, compared to preprocessor mode used in ccache 2.4, which is substantially less

Change-Id: I224de8e5bc53bbe18a7a02fc811324307cf3ab8c
Fix squisher
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

Change-Id: Ibd8e2b90455bd603a691c7b95466068b6fae212a