[TASK] Add generic virtual Ethernet source to stack projects

Add generic virtual Ethernet to generic stack sources with openMAC
Ethernet controller.

Change-Id: Id5152fcb3d99b3380f63d8bd3d202b20a2288357
[TASK] Enable virtual Ethernet in generic stack projects

Change-Id: Ia3859558ea4075f3a24698b4168a97ab9912086d
[TASK] Add user event callback to embedded MN demo

The user event callback can be used to handle stack events in the
application directly.

Change-Id: I057a40a99d937bb9ee680b3110df2496fca9e24b