Updates to allow F# to build without .Net 2.0 to being present on Mono

Removed FSharp.Core from lib/bootstrap/2.0
Moved all required bootstrap files from lib/bootstrap/2.0 to 4.0 folder
Updated build scripts to reflect the above change
Added conditions to the build script to only build a version 2.0 of FSharp.Core if
that version of mscorlib is available.

Mono plans to remove .Net2.0 and 3.5 assemblies in the next version.