sim: Add `--recursive` to submodule update
Now that mbed tls has its own submodules, add `--recursive` to the
sample invocation of submodule update.  Without this, building the
simulator produces difficult messages about `sha256.c` missing.
@@ -22,7 +22,7 @@ Dependent code
The simulator depends on some external modules. These are stored as
submodules within git. To fetch these dependencies the first time::
- $ git submodule update --init
+ $ git submodule update --init --recursive
will clone and check out these trees in the appropriate place.
