[build] Set $(TargetFrameworkRootPath)

[jonpryor]

@atsushieno and I are trying to figure out why src/Mono.Posix builds
for me on OS X but fails for him in a (relatively) clean Linux
environment:

CSC: error CS0518: The predefined type `System.Object' is not defined or imported

System.Object couldn't be found because System.Runtime.dll wasn't
being provided to the compiler.

Further investigation shows that the reason it built on OS X was
because OS X had a pre-existing system Xamarin.Android install, which
was being used to resolve framework directories:

# Works on OS X:
# Note: Mono.framework/External/xbuild-frameworks/MonoAndroid is a symlink
#   into Xamarin.Android.framework, i.e. the system Xamarin.Android install
Task "GetReferenceAssemblyPaths"
    Using task GetReferenceAssemblyPaths from Microsoft.Build.Tasks.GetReferenceAssemblyPaths, Microsoft.Build.Tasks.v4.0, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
    Looking for framework 'MonoAndroid,Version=v1.0' in root path '/Library/Frameworks/Mono.framework/External/xbuild-frameworks'
    Found framework definition list '/Library/Frameworks/Mono.framework/External/xbuild-frameworks/MonoAndroid/v1.0/RedistList/FrameworkList.xml' for framework 'MonoAndroid,Version=v1.0'
Done executing task "GetReferenceAssemblyPaths"

# Fails on Linux:
Target _GetReferenceAssemblyPaths:
    warning : Unable to find framework corresponding to the target framework moniker 'MonoAndroid,Version=v1.0'. Framework assembly references will be resolved from the GAC, which might not be the intended behavior.

Turns Out™, the error can be reproduced on OS X by "removing" the
system Xamarin.Android install:

$ cd /Library/Frameworks/Mono.framework/External/xbuild-frameworks
$ sudo mv MonoAndroid _MonoAndroid

Task "GetReferenceAssemblyPaths"
    Using task GetReferenceAssemblyPaths from Microsoft.Build.Tasks.GetReferenceAssemblyPaths, Microsoft.Build.Tasks.v4.0, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
    Looking for framework 'MonoAndroid,Version=v6.0' in root path '/Library/Frameworks/Mono.framework/External/xbuild-frameworks'
    Unable to find framework definition file '/Library/Frameworks/Mono.framework/External/xbuild-frameworks/MonoAndroid/v6.0/RedistList/FrameworkList.xml' for Target Framework Moniker 'MonoAndroid,Version=v6.0'
    Looking for framework 'MonoAndroid,Version=v6.0' in root path '/Library/Frameworks/Mono.framework/Versions/4.4.0/lib/mono/4.5/../xbuild-frameworks'
    Unable to find framework definition file '/Library/Frameworks/Mono.framework/Versions/4.4.0/lib/mono/4.5/../xbuild-frameworks/MonoAndroid/v6.0/RedistList/FrameworkList.xml' for Target Framework Moniker 'MonoAndroid,Version=v6.0'
    .../Microsoft.Common.targets:  warning : Unable to find framework corresponding to the target framework moniker 'MonoAndroid,Version=v6.0'. Framework assembly references will be resolved from the GAC, which might not be the intended behavior.
Done executing task "GetReferenceAssemblyPaths"

(Not sure why it's looking for v6.0`, but it can't find the
framework at all, so that's consistent with Linux.)

The fix is to set the $(TargetFrameworkRootPath) MSBuild property,
which is provided to the GetReferenceAssemblyPaths.RootPath
property, and allows controlling which locations are searched for
frameworks.

Unfortunately, $(TargetFrameworkRootPath) needs to be set to
bin\$(Configuration)\lib\xbuild-frameworks, which means
$(Configuration) needs to be set before
$(TargetFrameworkRootPath) is set. Furthermore, since we want to set
$(TargetFrameworkRootPath) in Configuration.props, this means that
we need to audit all locations which import Configuration.props to
ensure that $(Configuration) is set before the import.

Add $(TargetFrameworkRootPath) to Configuration.props, and audit
all uses of Configuration.props to ensure `$(Configuration) is set.