Include image descriptor of only the first boot image in vbmeta

Multiple boot images can be generated based on BOARD_KERNEL_BINARIES
but vbmeta would capture the image descriptor of only the first boot
image specified in BUILT_BOOTIMAGE_TARGET.

Bug : 158213759
Test: Compile multiple boot images and check if vbmeta.img
      includes image descriptor of only the first boot image
      specified in BUILT_BOOTIMAGE_TARGET

Change-Id: Ib70f65d98546a436da82656bc5c911d437618bb5