Move /tmp/ccache setup to base element

Commit c7d80dd (Cleanup mount points automatically) removed the unmount
of $TMP_MOUNT_PATH/tmp/ccache in run_d_in_target() and moved the
"rm /tmp/ccache" to elements/base/finalise.d/02-remove-ccache.  There
are two problems with this:

1) Not unmounting at the end of run_d_in_target() results in tmp/ccache
   being bind mounted muliple times on top of itself (three times, if you
   just run `disk-image-create base`).  It is eventually unmounted, but
   somehow the auto unmount code is confused, and tries to unmount it
   one more time than it was mounted, which results in an error like
   "umount: /tmp/image.THQkZxQa/mnt/tmp/ccache: not mounted".
   This doesn't actually break anything, but it's a little messy.

2) "rm /tmp/ccache" in elements/base/finalise.d/02-remove-ccache never
   succeeds in removing /tmp/ccache, because that hook is invoked by
   run_d_in_target(), *while* /tmp/ccache is mounted.

This present commit solves the above by moving the ccache setup glue out
of img-functions and into the base element's root.d.  This has the
following implications:

1) lib/img-functions is a little cleaner.

2) /tmp/ccache is available in the chroot during the root, extra-data,
   pre-install, install and post-install stages.  It is not available
   during block-device, finalise and cleanup stages as it will have been
   automatically unmounted by then.

3) /tmp/ccache won't be setup if you're building an image that doesn't
   include the base element.

Change-Id: Ief4c0a6f4ec622db6c6f652776215684178d8943
Merge "Move /tmp/ccache setup to base element"