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/subprocess.cc b/aos/starter/subprocess.cc
index 60f6b92..c36d59b 100644
--- a/aos/starter/subprocess.cc
+++ b/aos/starter/subprocess.cc
@@ -637,7 +637,7 @@
                             util::Top *top) {
   UpdateFileState();
 
-  CHECK_NOTNULL(builder);
+  CHECK(builder != nullptr);
   auto name_fbs = builder->CreateString(name_);
 
   const bool valid_pid = pid_ > 0 && status_ != aos::starter::State::STOPPED;