Update ART documentation for building from source

With trunk stable, we have different release configurations for building
a device. This updates the documentation for building ART from source
in a specific release configuration.

Test: N/A (documentation change)
Bug: 308188056

Change-Id: Ic4e96961f1fb1b3e6c0b072c0dd3860ff31708bc

1 files changed, 20 insertions, 3 deletions

diff --git a/build/README.md b/build/README.md
index 7b4e402376..4d9656fc10 100644
--- a/build/README.md
+++ b/build/README.md
@@ -75,12 +75,29 @@ module build on `master-art` above (b/172480617).
 
 2.  Ensure the ART Module is built from source:
 
+    Active development on ART module happens in `trunk_staging` release
+    configuration. Whenever possible, use this configuration for local development.
+
+    ```
+    lunch <product>-trunk_staging-<variant>
+    ```
+
+    Some release configurations use prebuilts of ART module. To verify whether a
+    particular release configuration uses ART prebuilts, run the following command
+
     ```
-    export ART_MODULE_BUILD_FROM_SOURCE=true
+    get_build_var RELEASE_APEX_CONTRIBUTIONS_ART
     ```
 
-    If this isn't set then the build may use prebuilts of the ART Module that
-    may be older than the sources.
+    If this returns a non-empty value, it usually means that this release
+    configuration uses ART prebuilts. (To verify, you can inspect the `contents` of
+    the `apex_contributions` Android.bp module reported by the above command.)
+
+    For troubleshooting, it might be desirable to build ART from source in that
+    release configuration. To do so, please modify the <release>.scl file and unset
+    the `RELEASE_APEX_CONTIRBUTIONS_ART` build flag. To determine which .scl files
+    are used in the current release configuration, please refer to
+    `out/release_config_entrypoint.scl`.
 
 3.  Build the system image the normal way, for example: