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.sh

@@ -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.
+#
+
+COMMONSOURCE="${BASH_SOURCE[0]}"
+while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
+  COMMONDIR="$( cd -P "$( dirname "$COMMONSOURCE" )" && pwd )"
+  COMMONSOURCE="$(readlink "$COMMONSOURCE")"
+  [[ $COMMONSOURCE != /* ]] && COMMONSOURCE="$COMMONDIR/$COMMONSOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
+done
+DIR="$( cd -P "$( dirname "$COMMONSOURCE" )" && pwd )"
+
+source "$DIR/_clang.sh"
+source "$DIR/_prettyprint.sh"
+source "$DIR/_rid.sh"
+
+# TODO: Replace this with a dotnet generation
+export TFM=dnxcore50
+export REPOROOT=$(cd $DIR/../.. && pwd)
+export OUTPUT_ROOT=$REPOROOT/artifacts/$RID
+export DNX_DIR=$OUTPUT_ROOT/dnx
+export DNX_ROOT=$DNX_DIR/bin
+export STAGE1_DIR=$OUTPUT_ROOT/stage1
+export STAGE2_DIR=$OUTPUT_ROOT/stage2
+export HOST_DIR=$OUTPUT_ROOT/corehost
+
+[ -z "$DOTNET_INSTALL_DIR" ] && export DOTNET_INSTALL_DIR=$REPOROOT/.dotnet_stage0/$RID
+[ -z "$DOTNET_BUILD_VERSION" ] && export DOTNET_BUILD_VERSION=0.1.0.0
+[ -z "$DOTNET_HOME" ] && export DOTNET_HOME=$STAGE2_DIR && export PATH=$STAGE2_DIR/bin:$PATH
+[ -z "$CONFIGURATION" ] && export CONFIGURATION=Debug   
+
+# if the conditional set is the last command in this file AND 
+# the condition is false then the program exits here. Seems like
+# a bug in BASH... or indication that there is more to learn about Linux
+# the below line is a hack that works around this issue
+a=1+1