ccache: Make the cache directory after ccache is built

This ensures that the cache directory (and all of its ancestor
directories) exist. This is a nice thing to do because, if the parent
of the cache directory doesn't exist, then ccache will complain that
it cannot create the cache directory, causing the build to fail.

[Peter: drop BR2_CCACHE conditional, use POST_INSTALL hook]
Signed-off-by: Dan Moulding <dan.moulding@rackwareinc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>