vendor_kernel_boot: Update releasetools for vendor_kernel_boot

add_img_to_target_files.py will re-make image and build a new
signed vbmeta.img with new images in target-file.
We need to add vendor_kernel_boot into AVB signing list and make it
support that image.

Test: make dist -j110 and check vbmeta.img
Bug: 214409109
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: Id07433f3dc33f95e2edd49de890f1e098cb9ef31