Merge pull request #6 from carlroth/master

More wonky DAC cable support, see PAN-1111
Use correct field.
New onlp_sysi_onie_info_get() interface.

Platforms without access to an ONIE-formatted eeprom can now
provide this vector to return the onie information directly.
Don't output an error message. The MISSING return code is sufficient
to allow a context-sensitive decision.