Add missing prefixes in default sysconfig file
    
    For arches we do not exlictly support, kdumpctl will fail with errors because
    kernel can not be found. It is caused by there's no proper KDUMP_IMG prefix
    so kernel image can not be found.
    
    Kexec/kdump functionality may simply work, so let's add those prefix in
    default sysconfig file thus one can test and use kexec/kdump in Fedora.
    
    Signed-off-by: Dave Young <dyoung@redhat.com>
    Reviewed-by: Pratyush Anand <panand@redhat.com>