device/odroid-u2: add device

Odroid-u2/u3 is based on exynos4412 and is well supported in mainline linux. No extra patches are added to start with. A wiki page has been created.

Works with both linux-lts and linux-postmarketos-exynos4. Linux-lts requires a bunch of modules to be added to initfs so that drivers for eMMC and sdcard are probed and file system is found.