testimage.bbclass: Don't require an image manifest

testimage.bbclass: Don't require an image manifest

Sometimes an "image" may not actually have a manifest file such as when
using a "baremetal kernel". This change allows for  a user to set a
IMAGE_NO_MANIFEST flag to 1 in order to inform the code that there is no
corresponding manifest that should exist for an image.

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>