gbl: efi: src: android_boot.rs: make vendor boot partition optional

On some devices, the vendor boot partition is not necessary. All
the necessary stuff is put in boot image V2.

Change-Id: Id4b91cc2f8001a8bf414042cced746e66df8cf73
Signed-off-by: Ying-Chun Liu (PaulLiu) <liupa@google.com>