Build: add default CCACHE_DIR and mkdir when the directory doesn't exist.

In Mokee Opensource, the final CCACHE_DIR is "[original CCACHE_DIR]/mk_[product]".
Ccache will fail to work if CCACHE_DIR is unset, or the directory "[original CCACHE_DIR]" doesn't even exist.

Maybe we can change it into a option to ask.

Change-Id: Iead142d2de32853a83715c23e2c333bc443698e7