image.bbclass, kernel.bbclass: create warning file about deleting deploydir files

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].

(From OE-Core rev: 4316c5e9eec67d71f4699102a63a5f968bd8d1d9)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>