Mynewt: image list command doesn't work for split images

[ccollins476ad]

Mynewt has the concept of "split images (http://mynewt.apache.org/latest/os/modules/split/split.html). Unlike for normal images, the split image state is stored in user flash, not in MCUboot trailers.

When mynewt contained its own management code (i.e., before switching to mcumgr), the split image state was properly indicated in an "image list" response. This functionality was lost with the transition to mcumgr.