Fix creating header v3 vendor-boot.img
requested to merge deathmist/halium-generic-adaptation-build-tools:header-v3-vendor-boot into halium-12
v3 only had a single vendor ramdisk with no fragments like v4, so adjust mkbootimg args accordingly as needed.
See also: https://source.android.com/docs/core/architecture/bootloader/boot-image-header#header-v4