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/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index 2882daf..8891c17 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -66,16 +66,19 @@
 
 static struct area_desc *flash_areas;
 
-void *(*mbedtls_calloc)(size_t n, size_t size);
-void (*mbedtls_free)(void *ptr);
+#ifdef MCUBOOT_SIGN_RSA
+int mbedtls_platform_set_calloc_free(void * (*calloc_func)(size_t, size_t),
+                                     void (*free_func)(void *));
+#endif
 
 int invoke_boot_go(struct area_desc *adesc)
 {
     int res;
     struct boot_rsp rsp;
 
-    mbedtls_calloc = calloc;
-    mbedtls_free = free;
+#ifdef MCUBOOT_SIGN_RSA
+    mbedtls_platform_set_calloc_free(calloc, free);
+#endif
 
     flash_areas = adesc;
     if (setjmp(boot_jmpbuf) == 0) {