scripts: west build: default build.pristine to auto

I've repeatedly seen that people are not aware of the existence of
this configuration option.

I've been using build.pristine=auto daily for years and never had any
problems. I've also asked around on Slack a couple of times over
various points to see if anybody objects to making this change. Nobody
has, so let's just turn it on by default.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>