Added minor edits to clarify that we are talking about an
image type.
(From yocto-docs rev: 14c906aa539d127c88d71d7fb3e628fc225cbd52)
Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>