sim: Validate the SHA256 verification

Instead of stubbing out the image validation code, compile it, and add
the SHA256 TLV to the buffer.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..960139a
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "sim/mbedtls"]
+	path = sim/mbedtls
+	url = https://github.com/ARMmbed/mbedtls