Currently, ccache is used if it is present. When building from scratch it gives
no performance improvement and creates a ton of empty directories even when its
not in use.
This change moves ccache support to a bbclass file which the user can choose to
enable. This should make builds more determinstic and make it easier/clearer
to the end user when its being used and when it is not.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
