actool/aci: simplify and improve build workflow

Requiring users to specify an `-app-manifest` [sic] made some sense when
we also had the concept of filesets, but is now of dubious utility. It
seems simpler to simply require people to pass a valid Image Layout to
`actool build`.

- Introduce --no-compression flag to `actool build` to allow users to
  disable the automatic gzip compression
- Handle errors while walking/building the tar. Previously
  this was doing things like attempting to copy the contents of a
  directory as a file into the tar, and silently failing.
- Convert all lingering references to app-manifests to image manifests
- Use shared consts for the locations of the image manifest/rootfs in
  the layout
- Remove relative-rootfs notion from the tar walker (and command line)
  since this no longer applies if users must supply an Image Layout
- Properly close file handles during validation of an Image Layout
  directory
- Remove deprecated/unused validateImageManifest function