GKI targets, e.g., `gki_arm64` or `gki_x86_64` have only
boot-*.img files without a ramdisk. It has no other images,
e.g., init_boot.img, system.img, etc.
The current build system assumes that the ramdisk is either
in a boot.img or in a init_boot.img, which is not true for
those GKI targets. Adding a new flag to support building
boot-*.img without a ramdisk while not building an init_boot.img.
Bug: 220834917
Test: build and `unpack_bootimg`
Change-Id: I789343c3e3d9ff0c36c0e19680a9792bd31a1c9f
85578e0a9d