Include BRILLO_VENDOR_PARTITIONS in target zip generation

This change will enable the target zip to include partition defined in
BRILLO_VENDOR_PARTITIONS. This is necessary for OTA to support
partitions other than boot and system in ab_partitions.txt.

The target zip will include images in BRILLO_VENDOR_PARTITIONS if
defined, under VENDOR_IMAGES/ with path kept. Also any vendor partitions
defined in AB_OTA_PARTITIONS will be copied to IMAGES/ in the target zip.

BUG: 28623063
Change-Id: Ic479048dfb8ac506acf827865e784fcb0432a1d5