cmake: extra_flags: Use zephyr_get

This allows users to set `EXTRA_*FLAGS` variables when using sysbuild.

If these flags are provided in both local and global sysbuild scopes,
then only the local ones will be used for a given image. This is to
circumvent issues with mixing space-separated and `;`-separated lists.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>