[FEATURE] Add configuration option for setting default gateway

The systems default gateway will be set by openPOWERLINK if virtual
Ethernet is enabled. This may not be usefull if the system has
additional Ethernet interfaces. Therefore the configuration option
CONFIG_VETH_SET_DEFAULT_GATEWAY was added to disable setting of the
default gateway.

Change-Id: I5c3c47a76c8f90fc6124dec647a3ea90b6e6c5ba
Reviewed-on: http://brateggevoat1/gerrit/2197
Tested-by: Jenkins <jenkins@brateggevoat1.br-automation.co.at>
Reviewed-by: Wolfgang Seiss <wolfgang.seiss@br-automation.com>
Reviewed-by: Josef Baumgartner <josef.baumgartner@br-automation.com>