Add /postinstall partition when using the A/B updater.

The new top level directory /postinstall is used by the A/B updater to
mount the new partition and run a post-install program before rebooting
into that new system.

init.rc was extended to label this new directory since the initrd has
no extended attributes.

Bug: 27177071
TEST=`ls -laZ /` shows the /postinstall directory on edison-eng

Change-Id: I6cdee7eb1d61ac7d447528962a4fad1a7bbf138d

rootdir/Android.mk

@@ -70,6 +70,11 @@ ifdef BOARD_ROOT_EXTRA_SYMLINKS
     ; mkdir -p $(dir $(TARGET_ROOT_OUT)/$(word 2,$(p))) \
     ; ln -sf $(word 1,$(p)) $(TARGET_ROOT_OUT)/$(word 2,$(p)))
 endif
+# The A/B updater uses a top-level /postinstall directory to mount the new
+# system before reboot.
+ifeq ($(AB_OTA_UPDATER),true)
+  LOCAL_POST_INSTALL_CMD += ; mkdir -p $(TARGET_ROOT_OUT)/postinstall
+endif
 
 include $(BUILD_SYSTEM)/base_rules.mk
 

rootdir/init.rc

@@ -23,6 +23,9 @@ on early-init
     # Shouldn't be necessary, but sdcard won't start without it. http://b/22568628.
     mkdir /mnt 0775 root system
 
+    # Set the security context of /postinstall if present.
+    restorecon /postinstall
+
     start ueventd
 
 on init