Fixes: #2562

The `make prepare` **make** target and `msbuild /t:Prepare` MSBuild
target are used to "prepare" a `xamarin-android` checkout so that
the subsequent `make all` or `msbuild /t:Build` invocation will work.

The preparation step involves ensuring that all known required
dependencies exist or are provisioned (54a7a02), such as the build
utilities (`autoconf`/etc.), cross-compilers (MinGW), the Android SDK
and NDK, and building "enough" to allow `Xamarin.Android.sln` to
build (see also 7343965).

Unfortunately, the Glorious Idea™ of using MSBuild for significant
portions of this process is "baroque", making it difficult to
understand and maintain, and the requirement on `msbuild` means
that things break in really bizarre and obscure ways on Linux and
macOS if the installed `mono` version is "too old"...and the
determination of "too old" was being done via `mono` & `msbuild`!

Furthermore, there was duplication and repetition of various bits of
build configuration on Linux and macOS between Makefile and the
MSBuild project system, which made it easier to make mistakes.
(Then there's the occasional bizarro failures that would result from
attempting to evaluate `$(PRODUCT_VERSION)` within `make`; it would
fail when `MONO_LOG_LEVEL=info` was set.  Then there's...)

Instead of having `make prepare` be implemented as an unholy mess of
`make` and `msbuild`, introduce the new `build-tools/xaprepare` app.
`xaprepare` will be built and executed as part of `make prepare`,
and takes over responsibility for repo build preparation.

See also: build-tools/xaprepare/README.md

TODO: Windows support