Add boot_16k.img and dtbo_16k.img to target_files

During signing, we need to re-generate dev option OTAs, as the 4K boot
images are getting signed with a different key. To allow generating
incremental OTAs for boot OTAs, include 16K variant of the boot images.

Test: th
Bug: 339658378
Change-Id: I0aafd5f48dbefeb2b86fd8bd93f6ce4d3f28a3cd