Add packet driver for NE2K compatible

[mfld-fr]

As the Asix AX88796-L chip on my SBC is claimed to be NE2K compatible in its datasheet, implement a tiny packet driver that could be connected later as backend to ktcp.

For implementation guidelines:
http://www.spinics.net/lists/linux-8086/msg00722.html
http://www.spinics.net/lists/linux-8086/msg00719.html

[mfld-fr]

Implementation started in branch ne2k.

As ELKS does not provide MDIO routines as modern Linux, first step is to drive the PHY and manage the link status, the speed and the duplex mode. Second step is to drive the MAC.

[mfld-fr]

Implementation and testing next steps:

• Draft of low-level routines
• Draft of test program
• Read PHY registers and link status
• Internal DMA write / read loop
• Receive single packet
• Send single packet
• ARP responder
• Continuous packet stream