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/compile/compile-corehost.ps1

@@ -0,0 +1,37 @@
+#
+# 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.
+#
+
+. $PSScriptRoot\..\common\_common.ps1
+
+header "Building corehost"
+pushd "$RepoRoot\src\corehost"
+try {
+    if (!(Test-Path "cmake\$Rid")) {
+        mkdir "cmake\$Rid" | Out-Null
+    }
+    cd "cmake\$Rid"
+    cmake ..\.. -G "Visual Studio 14 2015 Win64"
+    $pf = $env:ProgramFiles
+    if (Test-Path "env:\ProgramFiles(x86)") {
+        $pf = (cat "env:\ProgramFiles(x86)")
+    }
+    & "$pf\MSBuild\14.0\Bin\MSBuild.exe" ALL_BUILD.vcxproj /p:Configuration="$Configuration"
+    if (!$?) {
+        Write-Host "Command failed: $pf\MSBuild\14.0\Bin\MSBuild.exe" ALL_BUILD.vcxproj /p:Configuration="$Configuration"
+        Exit 1
+    }
+
+    if (!(Test-Path $HostDir)) {
+        mkdir $HostDir | Out-Null
+    }
+    cp "$RepoRoot\src\corehost\cmake\$Rid\$Configuration\corehost.exe" $HostDir
+
+    if (Test-Path "$RepoRoot\src\corehost\cmake\$Rid\$Configuration\corehost.pdb")
+    {
+        cp "$RepoRoot\src\corehost\cmake\$Rid\$Configuration\corehost.pdb" $HostDir
+    }
+} finally {
+    popd
+}