i9: migrate to system-as-root layout

* This is used for custom Android 10 ROMs (LineageOS 17.0)
* NOT compatible with Android 10 GSIs with non-SaR layout.

BoardConfig.mk

@@ -63,3 +63,6 @@ TW_MAX_BRIGHTNESS := 255
 TW_DEFAULT_BRIGHTNESS := 120
 TWRP_INCLUDE_LOGCAT := true
 TARGET_USES_LOGD := true
+
+# System-as-root - for Android 10 (non-GSI, LineageOS 17.0) support
+BOARD_BUILD_SYSTEM_ROOT_IMAGE := true

recovery.fstab

@@ -1,6 +1,6 @@
 /boot		emmc	/dev/block/platform/bootdevice/by-name/boot
 /recovery	emmc	/dev/block/platform/bootdevice/by-name/recovery	flags=backup=1
-/system		ext4	/dev/block/platform/bootdevice/by-name/system
+/system_root	ext4	/dev/block/platform/bootdevice/by-name/system flags=display="System"
 /vendor         ext4    /dev/block/platform/bootdevice/by-name/vendor flags=backup=1
 /data		ext4	/dev/block/platform/bootdevice/by-name/userdata	flags=encryptable=/dev/block/platform/bootdevice/by-name/metadata
 /cache		ext4	/dev/block/platform/bootdevice/by-name/cache

recovery/root/init.recovery.mt6739.rc

@@ -15,6 +15,9 @@ on init
 
     setprop crypto.ready 1
 
+on fs
+    export ANDROID_ROOT /system_root
+
 on post-fs-data
 # Support A/B feature for boot region
     symlink /dev/block/mmcblk0boot0 /dev/block/platform/bootdevice/by-name/preloader_a