initial GKI boot.img support

The GKI boot.img should be generated alongside the GSI. To make this
possible, define a flag (TARGET_NO_VENDOR_BOOT) that can be used to
disable the generation of the vendor_boot image.

Bug: 138323550
Merged-In: I91181853b26815e68a2fa2216b68579780d034dc
Change-Id: I91181853b26815e68a2fa2216b68579780d034dc