[CFC/CFT] Automatic dependency tracking for FDT/OFW.

[Aleksandr Rybalko]

Hello fellow hackers,
At first sorry for cross-post, but change affect mainly ARM/MIPS/PPC.
But other platforms sometime use FDT/OFW too.

So, if you are developer and you ever try/make some tricks to make devices
to attach in a different order, from that defined in FDT/OFW data, them it
maybe interesting for you.

https://reviews.freebsd.org/D21478

Please review/comment/test.

Thanks a lot!