scripts: west: add build directory to runner configuration

Add the build directory to the central runner configuration.
This is commonly useful information.

The first place we can use it is to eliminate guessing the current
working directory when building objects to parse .config.

It's not necessary to modify the build system for this, so leave the
relevant command line flag among the general options.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>