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/poly1305.c b/library/poly1305.c
index 2b56c5f..6fd9044 100644
--- a/library/poly1305.c
+++ b/library/poly1305.c
@@ -404,7 +404,7 @@
         ctx->queue_len++;
 
         /* Pad with zeroes */
-        memset( &ctx->queue[ctx->queue_len],
+        mbedtls_platform_memset( &ctx->queue[ctx->queue_len],
                 0,
                 POLY1305_BLOCK_SIZE_BYTES - ctx->queue_len );