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

Conflicts:
	core/tasks/kernel.mk