Is there a way to get sysbuild options per-board?

[CarlMitchellKT]

I've got a couple of boards with different requirements: One uses an RSA key for code signing, one uses an Ed25519 key. I want to have a way to pick the correct SB_CONFIG_BOOT_SIGNATURE_* options based on board, but it seems like the SB_* options are global to all boards an app may be built with.

It'd also be nice to be able to build for the native_posix boards, thus omitting MCUBoot entirely, while having sysbuild be the default (west config build.sysbuild True).

Is there a cleaner way than a bunch shell script to pass the appropriate options based on the target?