config: add sparc64/net flavor
    
    A small kernel is required for network install, as OpenBoot 4 only
    reserves 10MB for a network image into which we have to fit both
    kernel and initrd. The "net" flavor throws out everything but disk,
    net drivers and basic I/O and is currently 2966 KB in size.
    
    Signed-off-by: Jan Engelhardt <jengelh@medozas.de>