devtool: build-image: delete bbappend at end of build

devtool: build-image: delete bbappend at end of build

Upon further reflection, it seems to me that this bbappend ought to just
be deleted at the end of the build. This keeps things simple; you never
have to remember to delete any files to get back to where you were
before with the image. This means we can also drop the slightly awkward
message reminding the user how to do that. I've also updated the test to
look at the image manifest to determine if the command has worked
instead of looking for the (now deleted) bbappend.

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