Continue improving the readability of overlay.c.  The previous patches
renamed identifiers.  This patch is split out from the previous patches
to make the previous patches easier to review.

Changes are:
  - minor code restructuring
  - some initialization of an overlay changeset occurred outside of
    init_overlay_changeset(), move that into init_overlay_changeset()
  - consolidate freeing an overlay changeset into free_overlay_changeset()

This patch is intended to not introduce any functional change.
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>