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
43c0082aa7