west: build: Add new pristine cmd-line and config option

Add a new command-line and build config option, `pristine`, that the
user can pass to `west build` or set in its configuration file(s) in
order to automatically trigger a pristine build on every build or
whenever west considers it required.

The option can take the following values:

- never: Never run the target
- always: Always run the pristine target before building
- auto: Run the pristine target when required

With `auto`, the pristine target will be run when running
west with an existing build folder containing a build system and:

- Selecting a different board from the one currently in the build system
- Selecting a different application from the one currently in the build
  system

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>