configs: correct cpu accounting configuration
    
    The shipped .config files all have CONFIG_TICK_CPU_ACCOUNTING and
    CONFIG_VIRT_CPU_ACCOUNTING_NATIVE set. The x86_64 .config files
    additionaly have CONFIG_VIRT_CPU_ACCOUNTING_GEN set.
    
    This is wrong because each .config should have just one of these three
    set. As a result the build generates
        [...]:warning: override: VIRT_CPU_ACCOUNTING_NATIVE changes choice state
    
    24 times and
        [...]:warning: override: VIRT_CPU_ACCOUNTING_GEN changes choice state
    
    4 times. So let's make sure the shipped .config files only have one of
    these three Kconfig macros set (ie, the one that's relevant for that
    .config target).
    
    Signed-off-by: Paul Bolle <pebolle@tiscali.nl>