ccache: allow dynamic selection of cache directory

The existing ccache infrastructure sets the cache directory hardcoded in the
ccache binary. As this directory was set to ~/.buildroot-ccache, the cache
is not necessarily local (e.g. in corporate environments the home directories
may be mounted over NFS.)
Previous versions of buildroot did allow to set the cache directory, but this
was also hardcoded (so you had to rebuild ccache to change it), plus that
support was removed.
See http://lists.busybox.net/pipermail/buildroot/2011-July/044511.html

 for
a discussion on this.

This patch modifies ccache to respect a new shell variable (exported from
the Makefile, based on a configuration option) instead of CCACHE_DIR.
The name CCACHE_DIR itself is already used by autotargets for the ccache
package.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>