Further tweaks to buildbot

- BF_BITNESS should be passed as a command line argument
- Made it so CUDA binaries and OSL compiled scripts would
  be installed regardless WITH_BF_PYTHON (which seems to
  be quite obvious)
- Disable overwrite install, so CUDA kernels installed by
  it's build target will be preserved when building blender
  itself.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53697 954f8c5b-7b00-dc11-b283-0030488c597c