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.

scripts/data_files/error.fmt

@@ -51,7 +51,7 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
     if( buflen == 0 )
         return;
 
-    memset( buf, 0x00, buflen );
+    mbedtls_platform_memset( buf, 0x00, buflen );
 
     if( ret < 0 )
         ret = -ret;