Fix 16k ota package build failures on pixel

boot_ota_16k.zip relies on build_boot_from_kernel_avb_enabled. But that
makefile function is guarded by $(BOARD_PREBUILT_BOOTIMAGE). Pixel
defined BOARD_PREBUILT_BOOTIMAGE, which causes
build_boot_from_kernel_avb_enabled to be undefined, resulting in build
failure.

Test: th
Bug: 293313353
Change-Id: I5b01e9bf7fedd9c508f476bc243d860dc48c5ebb