`tmux new -c` fails on tmux older than 1.9, when that flag was added.
We can omit the flag for older versions of tmux, and the working
directory gets set even without it.
(From OE-Core rev: c55c294be6f5119f4c58a4e7a0bc052904126569)
Signed-off-by: Peter Budny <pbbudny@amazon.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>