Toolchain: Add QEMU build script and improve documentation

Added a script to build QEMU from source as part of the Toolchain.
The script content could be in BuildIt.sh but has been put in
a seperate file to make the build optional.

Added PATH=$PATH to sudo calls to hand over the Toolchain's PATH
setup by UseIt.sh. This enabled the script's to use the QEMU
contained in the SerenityOS toolchain.

Deleted old documentation in Meta and replaced it by a new
documentation in the Toolchain folder.

Toolchain/BuildQemu.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+set -e
+
+# This file will need to be run in bash, for now.
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+echo "$DIR"
+
+TARGET=i686-pc-serenity
+PREFIX="$DIR/Local"
+SYSROOT="$DIR/../Root"
+
+QEMU300_MD5SUM="6a5c8df583406ea24ef25b239c3243e0"
+QEMU410_MD5SUM="cdf2b5ca52b9abac9bacb5842fa420f8"
+
+QEMU_VERSION="qemu-4.1.0"
+QEMU_MD5SUM="${QEMU410_MD5SUM}"
+
+echo PREFIX is "$PREFIX"
+echo SYSROOT is "$SYSROOT"
+
+mkdir -p "$DIR/Tarballs"
+
+source "$DIR/UseIt.sh"
+
+pushd "$DIR/Tarballs"
+
+    md5="$(md5sum $QEMU_VERSION.tar.xz | cut -f1 -d' ')"
+    echo "qemu md5='$md5'"
+    if [ ! -e "$QEMU_VERSION.tar.xz" ] || [ "$md5" != "$QEMU_MD5SUM" ] ; then
+        rm -f qemu-3.0.0.tar.xz
+        curl -O "https://download.qemu.org/$QEMU_VERSION.tar.xz"
+
+        if  [ "$md5" != "$QEMU_MD5SUM" ] ; then
+            echo "qemu md5 sum mismatching, please run script again."
+            exit 1
+        fi
+    else
+        echo "Skipped downloading $QEMU_VERSION"
+    fi
+
+    if [ ! -d "$QEMU_VERSION" ]; then
+        echo "Extracting qemu..."
+        tar -xf "$QEMU_VERSION.tar.xz"
+    else
+        echo "Skipped extracting qemu"
+    fi
+popd
+
+mkdir -p "$PREFIX"
+mkdir -p "$DIR/Build/qemu"
+
+if [ -z "$MAKEJOBS" ]; then
+    MAKEJOBS=$(nproc)
+fi
+
+pushd "$DIR/Build/"
+    pushd qemu
+        "$DIR"/Tarballs/$QEMU_VERSION/configure --prefix="$PREFIX" \
+                                                --target-list=i386-softmmu \
+                                                --enable-gtk || exit 1
+        make -j "$MAKEJOBS" || exit 1
+        make install || exit 1
+    popd
+popd