Is there a SQL Server 2008 R2 specific target for SSDT projects?
A 2008 and 2008R2 user database is schema compatible, hence one version for both in SSDT. In other words, there aren't any schema objects that you could add to a 2008R2 SSDT model that couldn't be created in 2008.

This isn't the same as either database version or database compatibility level.

Database version:


  The database version is a number stamped in the boot page of a
  database that indicates the SQL Server version of the most recent SQL
  Server instance the database was attached to. The database version
  number does not equal the SQL Server version.


Database compatibility level:


  The database compatibility level determines how certain database
  behaviors work. For instance, in 90 compatibility, you need to use the
  OUTER JOIN syntax to do an outer join, whereas in earlier
  compatibility levels, you can use *= and =*.