Build in the correct KOTD repository with multibuild
    (JSC-SLE#5501, boo#1211226, bsc#1218184)
    
    With multibuild setting repository flags is no longer supported for
    individual spec files - see
    https://github.com/openSUSE/open-build-service/issues/3574
    
    Add ExclusiveArch conditional that depends on a macro set up by
    bs-upload-kernel instead. With that each package should build only in
    one repository - either standard or QA.
    
    Note: bs-upload-kernel does not interpret rpm conditionals, and only
    uses the first ExclusiveArch line to determine the architectures to
    enable.