[build] Add make prepare-image-dependencies target.

We are investigating building Xamarin.Android atop
Visual Studio Team System (VSTS) in addition to our current Jenkins
setup, and one of the issues we're running into is "bootstrapping":
VSTS is configured to create a "fresh" VM for each build.

**Pro**: It should allow builds to be more reliable, as previous build
artifacts won't be present, and thus won't cause/hide errors.

**Con**: *Previous build artifacts are not present*. Previous build
artifacts such as *downloading and extracting* the Android NDK & SDK,
using `brew` to install dependencies, building MXE...

Ensuring that the dependencies are installed through `make prepare`
can be quite time consuming. What we want is a way to ensure that the
"build image" -- *what's already installed* when the VM boots --
contains all of our desired dependencies.

Furthermore, we *don't* want to have the responsible parties checkout
and build xamarin-android in order to determine what the dependencies
should be.

Attempt to square this circle by adding a new
`make prepare-image-dependencies` target, which processes
`@(AndroidSdkItem)`, `@(AndroidNdkItem)`, `@(AntItem)`, and
`@(RequiredProgram)` to create a `prepare-image-dependencies.sh`
script which will download and install the required dependencies.

The generated shell script does *not* take the state of the machine
running `make prepare-image-dependencies` into consideration. This
allows the target to be executed on one machine, and the output run
on another.

	$ make prepare-image-dependencies
	# creates `prepare-image-dependencies.sh`

*Note*: `make prepare-image-dependencies` does not currently deal
with MXE.  (Building MXE on the VSTS VM is *very* time consuming, so
it's something we need to take care of. It is not *yet* dealt with.)

Author: jonpryor

Makefile

@@ -115,6 +115,10 @@ ifeq ($(USE_MSBUILD),1)
 	done
 endif	# msbuild
 
+prepare-image-dependencies:
+	$(MSBUILD) $(MSBUILD_FLAGS) build-tools/scripts/PrepareImageDependencies.targets /t:PrepareImageDependencies \
+		/p:AndroidSupportedHostJitAbis=mxe-Win32:mxe-Win64
+
 include build-tools/scripts/BuildEverything.mk
 include tests/api-compatibility/api-compatibility.mk
 

build-tools/scripts/PrepareImageDependencies.targets

@@ -0,0 +1,49 @@
+<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+  </PropertyGroup>
+  <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\xa-prep-tasks.dll"   TaskName="Xamarin.Android.BuildTools.PrepTasks.ReplaceFileContents" />
+  <Import Project="..\..\Configuration.props" />
+  <Import Project="..\android-toolchain\android-toolchain.projitems" />
+  <Import Project="..\dependencies\dependencies.projitems" />
+  <Target Name="PrepareImageDependencies">
+    <ItemGroup>
+      <_Dir Include="sdk" />
+      <_Dir Include="ndk" />
+      <_Dir Include="ant" />
+    </ItemGroup>
+    <ItemGroup>
+      <_Package
+          Condition=" '%(HostOS)' == '$(HostOS)' Or '%(HostOS)' == '' "
+          Include="@(AndroidSdkItem->'$(AndroidUri)/%(RelUrl)%(Identity) sdk/%(DestDir)')"
+      />
+      <_Package
+          Condition=" '%(HostOS)' == '$(HostOS)' Or '%(HostOS)' == '' "
+          Include="@(AndroidNdkItem->'$(AndroidUri)/%(RelUrl)%(Identity) ndk/%(DestDir)')"
+      />
+      <_Package
+          Condition=" '%(HostOS)' == '$(HostOS)' Or '%(HostOS)' == '' "
+          Include="@(AntItem->'$(AntUri)/%(RelUrl)%(Identity) ant/%(DestDir)')"
+      />
+    </ItemGroup>
+    <ItemGroup>
+      <_Brew
+        Condition=" '$(HostOS)' == 'Darwin' And '%(RequiredProgram.Homebrew)' != '' "
+        Include="@(RequiredProgram->'%(Homebrew)')"
+      />
+    </ItemGroup>
+    <PropertyGroup>
+      <_Packages>@(_Package->'%(Identity)', '
+')</_Packages>
+      <_Dirs>@(_Dir->'%(Identity)', '
+')</_Dirs>
+      <_Brews>@(_Brew->'%(Identity)', '
+')</_Brews>
+    </PropertyGroup>
+    <ReplaceFileContents
+        SourceFile="$(MSBuildThisFileDirectory)prepare-image-dependencies.sh.in"
+        DestinationFile="$(MSBuildThisFileDirectory)\..\..\prepare-image-dependencies.sh"
+        Replacements="@TOOLCHAIN_DIRS@=$(_Dirs);@PACKAGES@=$(_Packages);@BREWS@=$(_Brews)">
+    </ReplaceFileContents>
+  </Target>
+</Project>

build-tools/scripts/prepare-image-dependencies.sh.in

@@ -0,0 +1,72 @@
+#!/bin/bash -e
+set -o xtrace
+
+ARCHIVE_PATH="$HOME/android-archives"
+TOOLCHAIN_PATH="$HOME/android-toolchain"
+
+TOOLCHAIN_DIRS="
+@TOOLCHAIN_DIRS@
+"
+
+# format: URL path/under/$TOOLCHAIN_PATH
+PACKAGES="
+@PACKAGES@
+"
+
+BREW_PACKAGES="
+@BREWS@
+"
+
+function Download ()
+{
+	local url="$1"
+	local archive="$ARCHIVE_PATH/`basename "$url"`"
+	if [ -f "$archive" ]; then
+		return
+	fi
+	curl -o "$archive" "$url"
+}
+
+function Install ()
+{
+	local url="$1"
+	local subdir="$2"
+
+	local file=`basename "$url"`
+	local archive="$ARCHIVE_PATH/$file"
+	local targetdir="$TOOLCHAIN_PATH/$subdir"
+
+	local flagdir="$TOOLCHAIN_PATH/$(echo $subdir | sed 's#/.*$##g')"
+	local flagfile="$flagdir/.$file"
+
+	mkdir -p "$targetdir"
+	if [ ! -f "$flagfile" ]; then
+		local extractdir=`mktemp -d`
+		unzip -d "$extractdir" "$ARCHIVE_PATH/$file"
+		mv "$extractdir"/*/* "$targetdir"
+		touch "$flagfile"
+		rm -Rf "$extractdir"
+	fi
+}
+
+echo "$TOOLCHAIN_DIRS" | while read line ; do
+	if [ -z "$line" ]; then
+		continue
+	fi
+	rm -Rf "$TOOLCHAIN_PATH/$line"
+done
+
+echo "$PACKAGES" | while read line ; do
+	if [ -z "$line" ]; then
+		continue
+	fi
+	Download $line
+	Install $line
+done
+
+echo "$BREW_PACKAGES" | while read line ; do
+	if [ -z "$line" ]; then
+		continue
+	fi
+	brew install $line
+done