Add DESCRIPTION to each image bb file according to poky reference manual for Hob2 use later.
(From OE-Core rev: 05a7c022e613f35de8ce47bb667140a7ce47fcea)
Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>