Move mbedtls submodule from sim to ext

Move the external mbedtls submodule out of the sim/mcuboot-sys directory
into the ext directory.  This will allow the same copy of mbed TLS to be
used by other board support packages, instead of having to make a
duplicate clone.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/.gitmodules b/.gitmodules
index 4a84459..2b5f02c 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,3 @@
 [submodule "sim/mbedtls"]
-	path = sim/mcuboot-sys/mbedtls
+	path = ext/mbedtls
 	url = https://github.com/ARMmbed/mbedtls