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/pkcs5.c b/library/pkcs5.c
index a517778..e551547 100644
--- a/library/pkcs5.c
+++ b/library/pkcs5.c
@@ -231,7 +231,7 @@
     unsigned char *out_p = output;
     unsigned char counter[4];
 
-    memset( counter, 0, 4 );
+    mbedtls_platform_memset( counter, 0, 4 );
     counter[3] = 1;
 
 #if UINT_MAX > 0xFFFFFFFF