tools/litex_json2dts_linux.py: adding ip= bootarg when localip and remoteip are provided

[trabucayre]

Configuring network interface may be done by three different ways:

1. using ifconfig to configure interface and route for gateway
2. updating */etc/network/interfaces with all required informations
3. using ip= bootargs (requires to have a kernel configured with CONFIG_IP_PNP=y

First option needs a user's manual intervention and must be done for each boot. Second is hardcoded and this file must be updated when user change ip configuration.

Last option is more easy since, when ethernet is enabled, json contains all required informations to fill the ip= bootargs.

[enjoy-digital]

Thanks @trabucayre!