releasetools: Fix the rebuilding of vbmeta.img.

If only vbmeta.img is missing from the target_files.zip,
'add_img_to_target_files.py -a' will fail to re-create vbmeta.img.

++++ vbmeta ++++

Traceback (most recent call last):
  File "./build/make/tools/releasetools/add_img_to_target_files.py", line 693, in <module>
    main(sys.argv[1:])
  File "./build/make/tools/releasetools/add_img_to_target_files.py", line 687, in main
    AddImagesToTargetFiles(args[0])
  File "./build/make/tools/releasetools/add_img_to_target_files.py", line 584, in AddImagesToTargetFiles
    boot_contents = boot_image.WriteToTemp()
AttributeError: 'NoneType' object has no attribute 'WriteToTemp'

Because it has skipped loading the boot.img, which leads to the above error.

Bug: 63456822
Test: As follows:
  $ zip -d target_files.zip vbmeta.img
  $ add_img_to_target_files.py -a target_files.zip
Test: `m dist` on aosp_arm64-userdebug
Change-Id: Ieba5f0b6848c3ec5d8bcfd24d48cc5af8b39f06c