Add target "nsa325" to main makefile

Creates an U-Boot kernel image containing the compressed linux kernel and the accompanying NSA 325 OF dtb.
Add board suport for ZyXEL NSA325 (Flattened Device Tree)

Detection of the ethernet interface using OF dt didn't work, so let's initialize it statically.
Add device tree for ZyXEL NSA325 2-Bay Power Plus Media Server

Tested and works:
- All LEDs (SATA1/SATA2/Copy Green/Red, System Green/Orange, USB Green)
- Network (Booting with NFS as root file system works)
- Shutdown using "power off" GPIO
- Serial interface
- RTC (pcf8563)
- USB 2.0 (with "usb power enable" GPIO)

Untested:
- SATA
- NAND
- Buttons (Power, Copy, Reset)
- USB 3.0 (xHCI controller)
- MCU