os-release.d: Add system_id.

This is the app id of the SoM stored in the system.

Test: adb shell cat /etc/os-release.d/system_id
Change-Id: Iba7ac028ffa7402f06fbf9c4a3a077eb3414940e
(cherry picked from commit d41148b6c8d59d4f35711028248fc644428ddc57)
diff --git a/brillo_config/Android.mk b/brillo_config/Android.mk
index 7c1dd6c..fb0cd75 100644
--- a/brillo_config/Android.mk
+++ b/brillo_config/Android.mk
@@ -36,6 +36,20 @@
 	$(hide) mkdir -p $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)
 	ln -sf /oem/$(OSRELEASED_DIRECTORY)/product_id $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)
 
+include $(CLEAR_VARS)
+LOCAL_MODULE := system_id
+LOCAL_MODULE_CLASS := ETC
+LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/$(OSRELEASED_DIRECTORY)
+include $(BUILD_SYSTEM)/base_rules.mk
+
+# Attempt to populate the system id from a file in the product path.
+LOADED_BRILLO_SYSTEM_ID := $(call cfgtree-get-if-exists,brillo/system_id)
+
+$(LOCAL_BUILT_MODULE): BRILLO_SYSTEM_ID ?= "$(LOADED_BRILLO_SYSTEM_ID)"
+$(LOCAL_BUILT_MODULE):
+	$(hide) mkdir -p $(dir $@)
+	echo $(BRILLO_SYSTEM_ID) > $@
+
 
 include $(CLEAR_VARS)
 LOCAL_MODULE := product_version