ODROID-HC2 DTB workaround

[yuvadm]

The ODROID-HC2 boards are starting to hit the market, and the XU3 images should theoretically work on them.

There is one workaround needed in order to boot, if you follow the log at https://gist.github.com/yuvadm/218167e3 ... 7be61ed351 you can see there's a DTB file missing due to an `unknown` board.

To workaround this issue simply copy over `/boot/dtbs/exynos5422-odroidxu4.dtb` to `/boot/dtbs/exynos5422-odroidunknown.dtb`

A proper fix for this might be in place, but for now this is good enough.