add new target 'oxnas'

This is the oxnas target previously developed at
http://gitorious.org/openwrt-oxnas
Basically, this consolidates the changes and addtionas from
http://github.org/kref/linux-oxnas
into a new OpenWrt hardware target 'oxnas' adding support for
PLX Technology NAS7820/NAS7821/NAS7825/...
formally known as
Oxford Semiconductor OXE810SE/OXE815/OX820/...

For now there are 4 supported boards:
Cloud Engines Pogoplug V3 (without PCIe)
fully supported

Cloud Engines Pogoplug Pro (with PCIe)
fully supported

MitraStar STG-212
aka ZyXEL NSA-212,
aka Medion Akoya P89625 / P89636 / P89626 / P89630,
aka Medion MD 86407 / MD 86805 / MD 86517 / MD 86587
fully supported, see http://wiki.openwrt.org/toh/medion/md86587

Shuttle KD-20
partially supported (S-ATA driver lacks support for 2nd port)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43388 3c298f89-4303-0410-b956-a3cf2f4a3e73