network-settings: cleanup MAC Adress and hostname code
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Allow to 'unset' the mac address and serial number with sconfig serial 0/mac 0.
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Return N/A for all zero and prefix + zero mac address.
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Don't write an unset hostname.
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Add fallback code for hostname:
- SNE... as set
- version/modulename
- trizeps / myon from fuses
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Reuse more checkcode (some checks have been done in network config only, some only in conf-reader. Try to unify this.