lineage: Don't enforce specifying image name on prebuilt kernel

- The goal of specifying kernel image name was to whether kernel needs
  appended dtb or not, which is not something required if the kernel is
  prebuilt

Ref:
[0]: Commit edc1cebc

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>
Change-Id: I6b43756a2eac59729a67b5f80dac112f8b6bcc5b