[build] Install mono in make prepare-image-dependencies

I have been told that the `prepare-image-dependencies.sh` script
should also install `mono`, to ensure that the required mono exists.
Add a `@PKG_URLS@` variable to `prepare-image-dependencies.sh.in` to
support downloading and installing our minimum specified mono version.

Additionally, I discovered that `xbuild` bugs are still around; I just
keep forgetting their specifics. Case in point:
[Jenkins build #712][xa712] uploaded
[`prepare-image-dependencies.sh`][sh], and it's not good:

[xa712]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/
[sh]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/712/Azure/processDownloadRequest/xamarin-android/prepare-image-dependencies.sh

	https://dl.google.com/android/repository/build-tools_r26.0.1-macosx.zip sdk/build-tools\$(XABuildToolsFolder)
	...
	https://dl.google.com/android/repository/android-15_r03.zip sdk/platforms\android-15

MSBuild properties contained within item metadata -- specifically
`%(AndroidSdkItem.DestDir)` -- aren't fully expanded, resulting in the
above value of `$(XABuildToolsFolder)`, which *should* be `26.0.1`.
Similarly, some install paths are using `\` (backslash) when they
should contain `/` (forward slash).

Jenkins has been updated to explicitly use `msbuild`:

	$ make prepare-image-dependencies MSBUILD=msbuild

However, `\r` characters now appear in
`prepare-image-dependencies.sh`. Fix this by having the
`PrepareImageDependencies` target emit the MSBuild-generated file into
an intermediate file, then fix it up using **tr**(1).

Finally, correct the `brew install` command so that any errors are
ignored, for if a brew package is already installed, re-installing it
will result in an error.

Author: jonpryor

Makefile

@@ -116,6 +116,7 @@ endif	# msbuild
 prepare-image-dependencies:
 	$(MSBUILD) $(MSBUILD_FLAGS) build-tools/scripts/PrepareImageDependencies.targets /t:PrepareImageDependencies \
 		/p:AndroidSupportedHostJitAbis=mxe-Win32:mxe-Win64
+	cat bin/Build$(CONFIGURATION)/prepare-image-dependencies.sh | tr -d '\r' > prepare-image-dependencies.sh
 
 include build-tools/scripts/BuildEverything.mk
 include tests/api-compatibility/api-compatibility.mk

build-tools/scripts/PrepareImageDependencies.targets

@@ -32,18 +32,26 @@
         Include="@(RequiredProgram->'%(Homebrew)')"
       />
     </ItemGroup>
+    <ItemGroup>
+      <_DarwinPkg
+        Condition=" '$(HostOS)' == 'Darwin' And '%(RequiredProgram.Identity)' == 'mono' "
+        Include="%(RequiredProgram.DarwinMinimumUrl)"
+      />
+    </ItemGroup>
     <PropertyGroup>
       <_Packages>@(_Package->'%(Identity)', '
 ')</_Packages>
       <_Dirs>@(_Dir->'%(Identity)', '
 ')</_Dirs>
       <_Brews>@(_Brew->'%(Identity)', '
 ')</_Brews>
+      <_DarwinPkgs>@(_DarwinPkg->'%(Identity)', '
+')</_DarwinPkgs>
     </PropertyGroup>
     <ReplaceFileContents
         SourceFile="$(MSBuildThisFileDirectory)prepare-image-dependencies.sh.in"
-        DestinationFile="$(MSBuildThisFileDirectory)\..\..\prepare-image-dependencies.sh"
-        Replacements="@TOOLCHAIN_DIRS@=$(_Dirs);@PACKAGES@=$(_Packages);@BREWS@=$(_Brews)">
+        DestinationFile="$(MSBuildThisFileDirectory)\..\..\bin\Build$(Configuration)\prepare-image-dependencies.sh"
+        Replacements="@TOOLCHAIN_DIRS@=$(_Dirs);@PACKAGES@=$(_Packages);@BREWS@=$(_Brews);@PKG_URLS@=$(_DarwinPkgs)">
     </ReplaceFileContents>
   </Target>
 </Project>

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

@@ -17,6 +17,10 @@ BREW_PACKAGES="
 @BREWS@
 "
 
+PKG_URLS="
+@PKG_URLS@
+"
+
 function Download ()
 {
 	local url="$1"
@@ -68,5 +72,14 @@ echo "$BREW_PACKAGES" | while read line ; do
 	if [ -z "$line" ]; then
 		continue
 	fi
-	brew install $line
+	brew install $line || true
+done
+
+echo "$PKG_URLS" | while read line ; do
+	if [ -z "$line" ]; then
+		continue
+	fi
+	Download $line
+	pkg=`basename "$line"`
+	sudo installer -pkg "$ARCHIVE_PATH/$pkg" -target /
 done