recovery: deprecate check for ro.build.system_root_image

If there is a "/system" in the fstab, then can not be a
system as root image.

Test: compile
Bug: 109821005
Change-Id: I2c852dcbdcf6de437d39039937799feeef949516
diff --git a/recovery.cpp b/recovery.cpp
index 7cc344b..3ea282f 100644
--- a/recovery.cpp
+++ b/recovery.cpp
@@ -78,6 +78,7 @@
 static constexpr const char* DATA_ROOT = "/data";
 static constexpr const char* METADATA_ROOT = "/metadata";
 static constexpr const char* SDCARD_ROOT = "/sdcard";
+static constexpr const char* SYSTEM_ROOT = "/system";
 
 // We define RECOVERY_API_VERSION in Android.mk, which will be picked up by build system and packed
 // into target_files.zip. Assert the version defined in code and in Android.mk are consistent.
@@ -852,12 +853,12 @@
       }
       case Device::MOUNT_SYSTEM:
         // the system partition is mounted at /mnt/system
-        if (android::base::GetBoolProperty("ro.build.system_root_image", false)) {
+        if (volume_for_mount_point(SYSTEM_ROOT) == nullptr) {
           if (ensure_path_mounted_at("/", "/mnt/system") != -1) {
             ui->Print("Mounted /system.\n");
           }
         } else {
-          if (ensure_path_mounted_at("/system", "/mnt/system") != -1) {
+          if (ensure_path_mounted_at(SYSTEM_ROOT, "/mnt/system") != -1) {
             ui->Print("Mounted /system.\n");
           }
         }