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/md2.c b/library/md2.c
index 1c0b3df..fbec008 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -81,7 +81,7 @@
 
 void mbedtls_md2_init( mbedtls_md2_context *ctx )
 {
-    memset( ctx, 0, sizeof( mbedtls_md2_context ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_md2_context ) );
 }
 
 void mbedtls_md2_free( mbedtls_md2_context *ctx )
@@ -103,9 +103,9 @@
  */
 int mbedtls_md2_starts_ret( mbedtls_md2_context *ctx )
 {
-    memset( ctx->cksum, 0, 16 );
-    memset( ctx->state, 0, 46 );
-    memset( ctx->buffer, 0, 16 );
+    mbedtls_platform_memset( ctx->cksum, 0, 16 );
+    mbedtls_platform_memset( ctx->state, 0, 46 );
+    mbedtls_platform_memset( ctx->buffer, 0, 16 );
     ctx->left = 0;
 
     return( 0 );