[3554] Changes after review:
- Moved MAC operations to a new CfgMACSource class - Moved macros to HWaddr class - Added several unit-tests for MAC sources - MAC extraction code moved to separate function in Dhcpv6Srv
- Moved MAC operations to a new CfgMACSource class - Moved macros to HWaddr class - Added several unit-tests for MAC sources - MAC extraction code moved to separate function in Dhcpv6Srv