toolchain-scripts: Add parameters to toolchain_create_sdk_env_script.

To add some flexibility to setting up the paths for the toolchain,
add some parameters. This initial use will be in order to point at
the buildsystem toolchain copied in by copy_buildsystem.py.

Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>