Allow dtb image to be empty in vendor_boot image

Currently, building vendor_boot image will check size of
dtb image and abort if not specified. We want to allow this condition
and make user to build dtb whether in vendor_boot or vendor_kernel_boot.

Bug: 222429295
Bug: 214409109
Test: atest --host mkbootimg_test
Signed-off-by: Lucas Wei <lucaswei@google.com>
Change-Id: I7a96ddc700d2ca43bd1063fd634ef8af76e94005
3 files changed