supported-flag: fix build failures with SUSE_KERNEL_SUPPORTED=y
    
    Upstream commit 425937381ec (kbuild: re-run modpost when it is updated)
    added an expectation that the MODPOST variable would only point to the
    modpost executable and moved arguments to the modpost-args variable.
    
    Also removed some legacy stuff, like the assumption that MODVERDIR would
    exist (and then later creating it and using it) when the only two
    places we ever care about Module.supported being located are the directory
    in which an external module is being built and the current directory.