Clean up build scripts

Decompose into self-contained granular components
Provide reasonable defaults for cross cutting concerns, allowing for independent execution of steps
Start unifying Windows/Bash architecture

fix Bash CI scripts

dockerbuild.sh _common.sh path

Add missing restore-packages.sh

Copy/paste issues

Quote $SOURCE
fix .gitignore

PR Feedback

Merge in @SridarMS's work to avoid redownloading DNX

enabling build of dotnet-build

merge in @SridharMS's CentOS changes

Enable building FSC

enable restoring specific subdirectories

Fix dnx version check

Add missed dependency

Fix pathing to tests

Match Linux build version to Windows, fixing linux tests as a side effect.

workaround for coreclr#2215

fix pathing issue

disable building in docker

BUILD_IN_DOCKER was set, somehow...

fix headers

scripts/common/_clang.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) .NET Foundation and contributors. All rights reserved.
+# Licensed under the MIT license. See LICENSE file in the project root for full license information.
+#
+
+# Set up the environment to be used for building with clang.
+if which "clang-3.5" > /dev/null 2>&1; then
+    export CC="$(which clang-3.5)"
+    export CXX="$(which clang++-3.5)"
+elif which "clang-3.6" > /dev/null 2>&1; then
+    export CC="$(which clang-3.6)"
+    export CXX="$(which clang++-3.6)"
+elif which clang > /dev/null 2>&1; then
+    export CC="$(which clang)"
+    export CXX="$(which clang++)"
+else
+    error "Unable to find Clang Compiler"
+    error "Install clang-3.5 or clang3.6"
+    exit 1
+fi