Support AVB chain partitions for HASH descriptors

Commit Ida1a6da988c10d364b3ccdaa6c5d63e5264d1b27 allows specifying
AVB-related partitions to be probed in first-state mount, e.g.,
"avb=vbmeta_system" in the fs_mgr_flags will make init wait uevents
for vbmeta_system partition, which is required for AVB verification.

However, when specifying "avb=dtbo" to chain a /dtbo partition, first-stage
init will emit the following error as it assumes all partitions are
verified by Hashtree descriptors.

  init: [libfs_avb]Hashtree descriptor not found: dtbo

This change fixes this by skipping raw partitions for first-stage mounting,
where raw partitions should already be verified by bootloader with Hash
descriptors. Although first-stage init does not perform Hash verification,
it still reads all Hash/Hashtree descriptors to calculate then compare
the "VBMeta Digest" passed from bootloader.

  https://android.googlesource.com/platform/external/avb/#The-VBMeta-Digest

The following is an example of a fstab entry to chain /dtbo partition:

  /dev/block/bootdevice/by-name/dtbo  /dtbo  emmc  defaults  slotselect,avb=dtbo,first_stage_mount

Bug: 146935233
Test: enable /dtbo as a chain partition, then boot the device
Change-Id: I590f1d9b78ef89b5aef21cb7bf19fdb90b27f808