[coreboot] System gcc requirements

[Branden Waldner]

I recently looked in to why I couldn't build coreboot on one of my
systems any longer and I think I found the cause.

It looks like vboot uses features not available in gcc 6 on Debian Stretch.

I actually did manage to get it to build and work by commenting out
the offending gcc warning flags and a fallthrough switch attribute,
but that's kind of pointless.

Is there an expected minimal system gcc version and if so, is it
documented? I couldn't find it noted anywhere.

Branden