Replace --env with --source-dir

Remove the --env argument, which was compulsory for all subcommands, and
replace it with a --source-dir argument, only for those subcommands that
need it.

We do this for 2 reasons:
* Not all subcommands care about the source directory (e.g. llvm.py
  build won't need it)
* llvm.py should not have any knowledge about environments - that
  concept only makes sense for the helper scripts. llvm.py should
  instead receive very specific info about where the source and build
  directories are.

Change-Id: Iffaeef95559e8923bd883d5c51fed8c306287280
8 files changed