image_types: Add 7-Zip support in conversion types and commands

Add 7-Zip support in conversion types and commands. Default arguments of
7-Zip are compression level: 9, method: BZip2, extension: 7z. All these are
overridable from local.con for any other use case in user side. First goal
was to make an alternative zip conversion which can operate faster in
multi-thread systems (on many CPU cores), normal zip is just a single
thread compression. 7-Zip can do a compression in multi-thread processing,
so it is fine to use and speed up it and 7-Zip is supported also in many
platform to use.

Signed-off-by: Benjamin Szőke <egyszeregy@freemail.hu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>