pantah: add requirement for vendor_kernel_boot partition

Bug: 214409109
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: I756d2bbc730b5901bfe30d415bf519937e8b1ab0

board-info.txt

 require board=cloudripper|ravenclaw|cheetah|panther
+require partition-exists=vendor_kernel_boot