scripts: west sign: use edtlib for flash configuration

This command needs access to DT configuration, but can no longer
access it through BuildConfiguration since
9da1d41a.

Import edtlib and use that instead.

Fix up some other error handling and output issues while we're here to
make the script's behavior easier to inspect and debug.

Fixes: #20545
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>