sysbuild: provide a Kconfig setting name for boot signature choice

Kconfig choices should have setting name so that their default
choice can be adjusted, for example by a board.

It further prevents the `defined with a prompt outside the choice`
warning when trying to re-source a Kconfig tree where choices are
create without a setting name.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>