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

[jonpryor]

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.)

[akoeplinger]

I'd call it prepare-dependencies as it could be used for other things than just running on an image.

[jonpryor]

make prepare-dependencies is an interesting idea, but we already have a make prepare-deps target -- for "dependencies"! -- so this would be confusing.

How does make prepare-image-dependencies sound?

[akoeplinger]

@jonpryor not sure why we need image in the name, maybe make dependencies-script ? though I feel we're bikeshedding so whatever you prefer :)