net: Move the mac address and ethernet header types out of UIP.

These types aren't specific to ipv4 or ipv6 and can be shared by both. The MAC
address type is also used by the network interface drivers, so pulling it out
reduces the width of the interface between the drivers and the network stack.

Change-Id: I54f01705c63c11239898cd456dddf717d9466f54
11 files changed