Intel 1GBe NIC common code should be shared
Currently the Intel 1GBe drivers, e1000g and igb, use the same set of common code. Unfortunately in its current form this is duplicated across both repositories and makes syncing with our current upstream -- FreeBSD more difficult. This work moves the common code out of e1000g and igb directories and into a single location. It adds a README to describe how to update them. As a part of this changes are being made to make future syncing easier while a few minor improvements are going to try and find their way upstream if at all possible. As a part of this, and related 4090 and 4091 we'll be sycing with the upstream Intel code.
Note that this does not try and create a common module out of the two sources, both e1000g and igb have separately compiled instance of this common code. This is necessary based on the way that this code works.