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/arc4.c b/library/arc4.c
index b8998ac..eeebbdc 100644
--- a/library/arc4.c
+++ b/library/arc4.c
@@ -50,7 +50,7 @@
 
 void mbedtls_arc4_init( mbedtls_arc4_context *ctx )
 {
-    memset( ctx, 0, sizeof( mbedtls_arc4_context ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_arc4_context ) );
 }
 
 void mbedtls_arc4_free( mbedtls_arc4_context *ctx )