Replace memset() with mbedtls_platform_memset()

Steps:

1. sed -i 's/\bmemset(\([^)]\)/mbedtls_platform_memset(\1/g' library/*.c tinycrypt/*.c include/mbedtls/*.h scripts/data_files/*.fmt

2. Manually edit library/platform_util.c to revert to memset() in the
implementations of mbedtls_platform_memset() and mbedtls_platform_memcpy()

3. egrep -n '\<memset\>' library/*.c include/mbedtls/*.h tinycrypt/*.c
The remaining occurrences are in three categories:
    a. From point 2 above.
    b. In comments.
    c. In the initialisation of memset_func, to be changed in a future commit.
diff --git a/library/ecjpake.c b/library/ecjpake.c
index 3381c7c..cee56b4 100644
--- a/library/ecjpake.c
+++ b/library/ecjpake.c
@@ -1104,7 +1104,7 @@
     TEST_ASSERT( len == sizeof( ecjpake_test_pms ) );
     TEST_ASSERT( memcmp( buf, ecjpake_test_pms, len ) == 0 );
 
-    memset( buf, 0, len ); /* Avoid interferences with next step */
+    mbedtls_platform_memset( buf, 0, len ); /* Avoid interferences with next step */
 
     /* Client derives PMS */
     TEST_ASSERT( mbedtls_ecjpake_derive_secret( &cli,