Remove usage of CHECK_NOTNULL

We want to switch to absl logging instead of glog.  gtest and ceres are
going there, and we already have absl as a dependency.  ABSL doesn't
have CHECK_NOTNULL, and we can move things over in an easier to review
fashion.

Change-Id: Ifd9a11ec34a2357cec43f88dba015db9c28ed2cf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/starter/starterd_lib.cc b/aos/starter/starterd_lib.cc
index 6917bb5..79e564e 100644
--- a/aos/starter/starterd_lib.cc
+++ b/aos/starter/starterd_lib.cc
@@ -321,7 +321,7 @@
 }
 
 void Starter::AddChannel(const aos::Channel *channel) {
-  CHECK_NOTNULL(channel);
+  CHECK(channel != nullptr);
   std::unique_ptr<aos::ipc_lib::MemoryMappedQueue> queue =
       std::make_unique<aos::ipc_lib::MemoryMappedQueue>(
           shm_base_, FLAGS_permissions, event_loop_.configuration(), channel);