packaging: Minor changes to Makefile.

- Remove binaries from bin/ before copying them to avoid "text file is
  busy" error.
- Added variable "XTREEMFS_BINARIES_DIR" (points to "bin/").
- Removed unnecessary variable "XTFS_BINDIR".


git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3078 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534