support use of prebuilt bootable images

img_from_target_files now, with the -z flag, will produce an output
zip with only the bootable partitions (boot and recovery).

img_ and ota_from_target_files can take, instead of a simple
"target_files.zip", a name of the form
"target_files.zip+bootable_images.zip", where the second zip contains
bootable images that should be used instead of building them from the
target_files.zip.  (This should be the zip produced by the above -z
flag, perhaps with the images messed with in some way, such as by an
unnamed OEM's extra signature wrapper for their "secure boot"
process.)

Bug: 3391371
Change-Id: Iaf96dfc8f30e806ae342dcf3241566e76ae372d4