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