Adding a usb net drivers directory
Adjusting make file to include drivers/net/usb modules in build
Adjusting asix driver to use include/ipxe headers and removing legacy #ifndef
Adjusting dm9601 driver to use include/ipxe headers and removing legacy #ifndef
Adjusting pegasus driver to use include/ipxe headers
Adjusting include/ipxe/errfile.h to assign defines for asix, dm9601, pegasus drivers
Adjusting asix driver DBG format specifiers lx to zx
Adjusting dm9601.h header file to use include/ipxe headers
Adjusting dm9601.h includes, so that it doesn't include include/ipxe/usb.h a second time and try to redefine struct dm9601 a second time and cause a warning/error
Adjusting config/config.c file to pull in USB host stack, ifdefines in the src code does not appear to work, default Make target iPXE does not pull in USB device drivers, so only targeted builds like bin/asix.iso will produce valid images
Adjusting config/console.h to turn off inclusion of a serial console for debugging purposes
Adjusting parserom.pl, config.c, asix, dm9601, pegasus files to support build system USB_ROM macros, Removed ohci_hcd bus driver files because they compete with the uhci_hcd bus driver during linking - one way to override is to include the DEBUG=ohci_hcd parameter after make, but this is not a good work around, so will find a way to resolve ambiguity before adding the ohci_hcd back
README.usb file updated with current information
Fixed asix.c and dm9601.c so they would compile without warnings on 64bit Ubuntu 14.04 LTS Desktop
Fixed asix.c forgot to remove unused gpio0 variable and code that used it
Added new USB_ROM id for a Lenovo U2L 100-Y1 USB Ethernet adapter
Adding Apple USB->Ethernet adapter.

The Apple USB->Ethernet adapter appears with the following details according to linux:
```
[87658.231359] usb 3-2.4: new high-speed USB device number 30 using xhci_hcd
[87658.272752] usb 3-2.4: New USB device found, idVendor=05ac, idProduct=1402
[87658.272758] usb 3-2.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[87658.272763] usb 3-2.4: Product: Apple USB Ethernet Adapter
[87658.272766] usb 3-2.4: Manufacturer: Apple Inc.      
[87658.272769] usb 3-2.4: SerialNumber: 2D8CB1
[87658.633762] asix 3-2.4:1.0 eth1: register 'asix' at usb-0000:08:00.0-2.4, ASIX AX88772 USB 2.0 Ethernet, 48:d7:05:e9:40:52
[87663.795160] asix 3-2.4:1.0 eth1: link up, 100Mbps, full-duplex, lpa 0xCDE1
```
Merge pull request #1 from mithro/patch-1

Adding Apple USB->Ethernet adapter.