scripts/west sign: Use BuildConfiguration instead of load_dot_config

The ImgtoolSigner.sign() is passed a BuildConfiguration object instance
that holds all of Kconfig options that have been defined for a build;
yet it has been reading the .config file once again, with the
load_dot_config, to check some of these options.
The commit replaces the code that has been using the load_dot_config
obtained data with the code that uses the BuildConfiguration object.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>