Add target_board_auto soong config variable

It will be used to configure a compile time flag when building apexd for
auto products. See the attached bug for more context

Test: m
Bug: 241473698
Change-Id: I7e7ce3216e455ff44b9a24fec14d6ee384e97192
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index bb622f0..93c9c16 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -121,6 +121,10 @@
 
 $(call soong_config_set,art_module,source_build,$(ART_MODULE_BUILD_FROM_SOURCE))
 
+ifdef TARGET_BOARD_AUTO
+  $(call add_soong_config_var_value, ANDROID, target_board_auto, $(TARGET_BOARD_AUTO))
+endif
+
 # Ensure that those mainline modules who have individually toggleable prebuilts
 # are controlled by the MODULE_BUILD_FROM_SOURCE environment variable by
 # default.