Allow android_dt_dir to be set by bootconfig

The androidboot.android_dt_dir property is special, because it is loaded
to find out where to get the other DT properties from, and those DT
properties are supposed to override the cmdline/bootconfig ones. So, it
need special casing, and that special case lacked bootconfig support.

Bug: 173815685
Test: launch_cvd -extra_kernel_cmdline androidboot.android_dt_dir=/tmp
[..]
init: Using Android DT directory /tmp
[..]
Change-Id: Ie0958dd0a96394d65f6568653b754ea6f885212e
diff --git a/init/util.cpp b/init/util.cpp
index eab99d4..a40d104 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -376,6 +376,15 @@
             android_dt_dir = value;
         }
     });
+    // ..Or bootconfig
+    if (android_dt_dir == kDefaultAndroidDtDir) {
+        ImportBootconfig([&](const std::string& key, const std::string& value) {
+            if (key == "androidboot.android_dt_dir") {
+                android_dt_dir = value;
+            }
+        });
+    }
+
     LOG(INFO) << "Using Android DT directory " << android_dt_dir;
     return android_dt_dir;
 }