west: sign: Add Kconfig with application version

Adds a Kconfig option which controls the version of the application
to use when the image is signed using imgtool.

When an application VERSION file is present, the default value will be
identical to the application version, else it will be 0.0.0+0, but a
project may still decide another value, if it so prefers.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>