ccache: move environment variables to the configuration file

ccache: move environment variables to the configuration file

Move some environment variables to the configuration file, so there's
less happening in the class.

Max_size was removed so that the per-recipe caches don't grown without
limit. The default cache is 5GB.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>