[lr/initrd-ofw] initrd: add devicetree support

This adds capability to hand over the network configuration from OpenFirmware (and potentially other boot loaders with openfirmware support such as U-Boot) to NetworkManager.

It's done analogously to ACPI/iBFT. In fact, the same ip=ibft command line option is used, adding a more general ip=fw alias. This probably deserves some documentation, but I'm not adding any at this time.