image-layout: Replace "slash separated layout" with "directory structure"
image-layout: Replace "slash separated layout" with "directory structure"

Adjusting wording we've been carrying since 9bb56d8e (image-layout:
add an initial image layout spec, 2016-05-26, #605).

On Windows, an unpacked layout will use backslashes.  Layouts really
don't care what the directory separator is, and each access method
(filesytem access, tar unpacker, etc.) already has an unambiguous
separator character.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

jonboulle/image-spec,opencontainers/image-spec,jonboulle/image-spec,opencontainers/image-spec