Add instructions for building #3

This patch adds some basic instructions for building, as well as patches needed for Buildroot.

I've fixed the certs.sh instructions and tested the entire thing using the latest stable tree. The instructions are complete aside from installing dependencies

Okay, this PR is ready for feedback. Then I'll squash it down and rebase for merging. You can read it online at https://git.lumina-sensum.com/LuminaSensum/buildroot-MynaPlayer/blob/WIP_README/README.md

Xogium pointed out a few issues on IRC that I fixed, but there's two major ones I need to fix tomorrow:

• Instead of using Git repos, download tarballs for buildroot and easy-rsa
• Make clearer instructions for installation using rauc and dd

This looks perfect to me.

Thanks !

Add instructions for building