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/_common.ps1

@@ -0,0 +1,23 @@
+#
+# 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.
+#
+
+. $PSScriptRoot\_utility.ps1
+
+$Rid = "win7-x64"
+$Tfm = "dnxcore50"
+$RepoRoot = Convert-Path "$PSScriptRoot\..\.."
+$OutputDir = "$RepoRoot\artifacts\$Rid"
+$DnxDir = "$OutputDir\dnx"
+$DnxRoot = "$DnxDir\bin"
+$Stage1Dir = "$OutputDir\stage1"
+$Stage2Dir = "$OutputDir\stage2"
+$HostDir = "$OutputDir\corehost"
+$PackageDir = "$RepoRoot\artifacts\packages\dnvm"
+
+# Set reasonable defaults for unset variables
+setEnvIfDefault "DOTNET_INSTALL_DIR"  "$(Convert-Path "$PSScriptRoot\..")\.dotnet_stage0\win7-x64"
+setEnvIfDefault "DOTNET_BUILD_VERSION" "0.1.0.0"
+setPathAndHomeIfDefault "$Stage2Dir"
+setEnvIfDefault "CONFIGURATION" "Debug"