Hi, I haven't done much in recent weeks because the machine I was testing the build on (or 3 of them) all died in a short space of time.... Two of them due to the fact that I was using IDE hard disks out of the laptop caddy mount and when swapping them after imaging I misaligned the pins. Long story short motherboards tend not to like that (Bang!)
Anyway for info you cannot just build mediacenter-osmc you need to add lines to the code based on the ATV installer (i386). What you really want to do is to try and build the pc file system first because that's a pretty easy place to get started. After that point you need some grub skills to get booted.
Also the apt repo does not have the mediacenter-osmc (x64) package in in you need to build that afterwards but to do so you need to modify makefile, patch and build.sh
In the mean time until a good build is made my suggestion is that if you are desperate for a lightweight kodi you should just do a debian net install base system and then install x and kodi. You can make it look like OSMC and even build the latest kodi from source, its all good experience.