Update mbedtls submodule to 2.12.0

This also updates the RSA test to add extra build files for zeroize, and
updates to new calloc/free configuration style.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index a0c747c..2c28dbb 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -40,6 +40,8 @@
 
         conf.file("mbedtls/library/rsa.c");
         conf.file("mbedtls/library/bignum.c");
+        conf.file("mbedtls/library/platform.c");
+        conf.file("mbedtls/library/platform_util.c");
         conf.file("mbedtls/library/asn1parse.c");
     } else if sig_ecdsa {
         conf.define("MCUBOOT_SIGN_EC256", None);