[coreboot] Please clarify wiki on git vs releases

[Ian Kelling]

The release readme says to read http://www.coreboot.org/Build_HOWTO,
which says to use the latest git sources. The mainpage of the wiki is a
bit ambiguous, but also leads to that same link under getting
started. So, you've given the impression that the latest git sources are
what's recommended to users.

It seems a bit odd to recommend users to use latest git sources on
something that can easily cause their system to not boot.