Exclude vendor_boot in secondary payload

The secondary payload is used by factory OTA to speed up the first
boot of the device. The payload should only contain bootloaders
images and the precompiled apps in system_other. Remove vendor_boot
to reduce the secondary payload size.

Test: build a package with secondary payload
Change-Id: I3a9f6dda00a5aac038438065e2bc6483d71b5e95