Reland: Moving GSI-specific files to system_ext

Only common files can reside in system partition, other files
should be moved to the newly added system_ext partition.

Note that for GSI, it will be a single system.img that includes the
contents of product and system_ext partitions, under /system/product
and /system/system_ext, respectively. After moving skip_mount.cfg to
system_ext partition, it also needs a symlink file under system
partition:

  /system/etc/init/config -> /system/system_ext/etc/init/config

This allows Q-launched first-stage init (in /boot partition) continue
to use the same path when new GSI image is used.

Bug: 138281441
Test: build aosp_arm64-userdebug and boot it on crosshatch
Test: rm -rf out && build/soong/soong_ui.bash --make-mode \
      TARGET_PRODUCT=aosp_arm64 TARGET_BUILD_VARIANT=userdebug droid
Change-Id: Iae9f5fb688f49497563864eb882d5f0ae33c744a
diff --git a/CleanSpec.mk b/CleanSpec.mk
index a84e793..7ce4ed7 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -655,6 +655,10 @@
 $(call add-clean-step, rm -rf $(OUT_DIR)/target/product/generic*/*/product)
 $(call add-clean-step, rm -rf $(OUT_DIR)/target/product/generic*/*/system_ext)
 
+# Move GSI-specific files from /system to /system/system_ext
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/etc/init/init.gsi.rc)
+$(call add-clean-step, rm -rf $(PRODUCT_OUT)/system/etc/init/config/)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************