[docs] Remove stale reference to MDI

Change-Id: I69ef00aa1cb57797c9959cced281f1f7865787f3

[build] Simplify target ("project") names

The only projects supported now are Zircon, so having "zircon-"
at the front of every name is just more typing. There is only
one architecture associated with each platform name, and for
x86-64 there is only one platform. The old -> new mappings are:

zircon-pc-x86-64 -> x86
zircon-qemu-arm64 -> arm64
zircon-gauss-arm64 -> gauss
zircon-hikey960-arm64 -> hikey960
zircon-odroidc2-arm64 -> odroidc2

The old project names continue to work as aliases during
a transition period but they are deprecated.

Since all projects are Zircon (nee Magenta), the -m switch to
buildall is removed and the build-all-zircon script is deprecated.

Change-Id: Icf88e93b460df053be251c88d9dffd8c6da82518

[entropy][docs] Document jitterentropy tuning

Jitterentropy has four numeric parameters that can (and should) be
tuned for different targets. This document discusses how to do that.

Change-Id: I332eebcf0ee4c1d394b0c7098e06d864ee151932