scripts: re-work BuildConfiguration boolean handling

Set options that are definitely true or false to True or False in the
options dict. Add a 'getboolean' method that also allows a fallback to
False in case the option is not mentioned in .config due to unmet
dependencies. This allows calling code to just ask about the option
they are interested in, even if the .config file doesn't mention the
option at all.

Propagate this to users within the runners package and 'west sign',
taking advantage of the new build_conf property.

Rename the 'bcfg' internal variable in sign.py to 'build_conf' to
match other source files that use BuildConfiguration instances, to
make it easier to grep for users.

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