New xbps-src variable

Hi xtraeme,
Let me explain you why I decided to add it.
Let's assume, I'm running another distro and I downloaded xbps static binaries (with no <code>xbps.conf</code>), when I'll use <code>xbps-src binary-bootstrap</code>, it'll fail because the repository isn't specified.
With the new option in <code>xbps-src.conf</code>, you can specify a repo or a custom config file for example.
So, when <code>xbps-src</code> will use static <code>xbps-install</code>, the additional arguments will be added.

I tested it and it works flawlessly.