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
		
	
		
			
				
	
	
	
		
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This is the base element.
Almost all users will want to include this in their disk image build, as it includes a lot of useful functionality.
Notes:
- 
If you are getting warnings during the build about your locale being missing, consider installing/generating the relevant locale. This may be as simple as having language-pack-XX installed in the pre-install stage
 - 
This element ensures /tmp/ccache will be available in the chroot during the root, extra-data, pre-install, install and post-install stages. /tmp/ccache is unavailable during block-device, finalise and cleanup stages as it will have been automatically unmounted by then.