First-stage mount: avoid triggering a FATAL error

Related AOSP changes:
  1. https://android-review.googlesource.com/#/c/platform/system/core/+/405009/
  2. https://android-review.googlesource.com/#/c/platform/system/core/+/532637/

The second CL raises a FATAL error when it detects fstab-dt has no content
during first-stage mount. However, with the first CL, the fstab-dt entry
might be "skipped" when bootloader sets the status property to a value
other than "ok"/"okay". (e.g., to skip mounting /vendor on upgrading
devices which have no vendor partition).

Use LOG(INFO) when there is nothing to mount here. The later stages
should trigger a FATAL error when some important files in those
partitions are not available, e.g., SEPolicy files.

Bug: 78441220
Test: boot a device
Change-Id: Iae2f47d455679298bdb067d96b771a30c1a82e6f
Merged-In: Iae2f47d455679298bdb067d96b771a30c1a82e6f
(cherry picked from commit 8fe363f260)

init/init_first_stage.cpp

@@ -119,10 +119,7 @@ static bool inline IsRecoveryMode() {
 FirstStageMount::FirstStageMount()
     : need_dm_verity_(false), device_tree_fstab_(fs_mgr_read_fstab_dt(), fs_mgr_free_fstab) {
     if (!device_tree_fstab_) {
-        // The client of FirstStageMount should check the existence of fstab in device-tree
-        // in advance, without parsing it. Reaching here means there is a FATAL error when
-        // parsing the fstab. So stop here to expose the failure.
-        LOG(FATAL) << "Failed to read fstab from device tree";
+        LOG(INFO) << "Failed to read fstab from device tree";
         return;
     }
     // Stores device_tree_fstab_->recs[] into mount_fstab_recs_ (vector<fstab_rec*>)