Add a new config option USE_STATIC_NETWORKING

USE_STATIC_NETWORKING will prevent the detected config from being ignored when DHCP is also detected.  With `USE_DHCLIENT=yes` set, the behaviour is:
- Interfaces using DHCP are configured by `dhclient` if `USE_STATIC_NETWORKING=yes` is not present.
- Interfaces using DHCP are configured by `dhclient` if `USE_STATIC_NETWORKING=yes` is present.
- Interfaces not using DHCP are configured by 60-network-devices.sh if `USE_STATIC_NETWORKING=yes` is present.
Merge pull request #488 from mavit/447-use-static-networking

Add a new config option USE_STATIC_NETWORKING