of: overlay: rename identifiers to more reflect what they do

This patch is aimed primarily at drivers/of/overlay.c, but those
changes also have a small impact in a few other files.

overlay.c is difficult to read and maintain.  Improve readability:
  - Rename functions, types and variables to better reflect what
    they do and to be consistent with names in other places,
    such as the device tree overlay FDT (flattened device tree),
    and make the algorithms more clear
  - Use the same names consistently throughout the file
  - Update comments for name changes
  - Fix incorrect comments

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>