image.bbclass, kernel.bbclass: create warning file about deleting deploydir files
New users sometimes delete files in tmp/deploy/images assuming they will
be re-created just by re-running the build, and then are confused when
this doesn't work. To discourage people from doing this, create a file
called README_-_DO_NOT_DELETE_FILES_IN_THIS_DIRECTORY.txt in
tmp/deploy/images containing an explanation of what needs to be done to
re-create files there.
Fixes [YOCTO #1315] and [YOCTO #1346].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
