ccache: allow for BR2_CCACHE_DIR environment override

Allow the BR2_CCACHE_DIR .config option to be overriden by the
BR2_CCACHE_DIR env variable.
This is useful for big projects where in some cases the developers home
directory might be a NFS mount (slow) and real production builds aren't.

Update documentation accordingly as well.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>