fs_mgr_avb: allow top-level vbmeta struct to be in 'boot' partition

avb_ops->read_from_partition() should return
AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION if the requested partition does
not exist. Then libavb will try to load top-level vbmeta from /boot partition.

Two related changes in external/avb:

  https://android-review.googlesource.com/#/c/344907/
  https://android-review.googlesource.com/#/c/348731/

Bug: 35880930
Test: test AVB on bullhead with top-level vbmeta being at /boot
Change-Id: I80355ba5ae4d29a6f102714cd2f7c2550b238f27
diff --git a/fs_mgr/fs_mgr_avb_ops.cpp b/fs_mgr/fs_mgr_avb_ops.cpp
index dd8c7e7..2c14c9b 100644
--- a/fs_mgr/fs_mgr_avb_ops.cpp
+++ b/fs_mgr/fs_mgr_avb_ops.cpp
@@ -78,7 +78,7 @@
     // access. fs_mgr_test_access() will test a few iterations if the
     // path doesn't exist yet.
     if (fs_mgr_test_access(path.c_str()) < 0) {
-        return AVB_IO_RESULT_ERROR_IO;
+        return AVB_IO_RESULT_ERROR_NO_SUCH_PARTITION;
     }
 
     android::base::unique_fd fd(TEMP_FAILURE_RETRY(open(path.c_str(), O_RDONLY | O_CLOEXEC)));