bitbake.conf: Add CCACHE_DIR to BB_HASHBASE_WHITELIST

bitbake.conf: Add CCACHE_DIR to BB_HASHBASE_WHITELIST

The location of the ccache should not affect the sum in the sstate
files.

The end user should be free to use an external ccache via defining
CCACHE_DIR to a new value in local.conf or using the default per
package local ccache in the temp directory.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>