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.

!257
20 jobs for lr/initrd-ofw in 88 minutes and 58 seconds (queued for 2 seconds)