Teach logger about O_DIRECT

Plump the option down through aos.

This will let us (in a future change) detect whether the storage disk
should use O_DIRECT or not, as appropriate.

Change-Id: I2bc68606a4954460a3bcd61e5e649e122ebb1358
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/events/logging/log_writer.h b/aos/events/logging/log_writer.h
index f63ac3e..114e874 100644
--- a/aos/events/logging/log_writer.h
+++ b/aos/events/logging/log_writer.h
@@ -154,6 +154,7 @@
 
   // Shortcut to call StartLogging with a MultiNodeFilesLogNamer when event
   // processing starts.
+  // Doesn't try to use odirect.
   void StartLoggingOnRun(std::string base_name) {
     event_loop_->OnRun([this, base_name]() {
       StartLogging(std::make_unique<MultiNodeFilesLogNamer>(