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/cmac.c b/library/cmac.c
index 5d101e1..ada0535 100644
--- a/library/cmac.c
+++ b/library/cmac.c
@@ -450,7 +450,7 @@
     }
     else
     {
-        memset( zero_key, 0, MBEDTLS_AES_BLOCK_SIZE );
+        mbedtls_platform_memset( zero_key, 0, MBEDTLS_AES_BLOCK_SIZE );
 
         ret = mbedtls_cipher_cmac( cipher_info, zero_key, 128, key,
                                    key_length, int_key );