ohos-image.bbclass: Define dtb_boot_files and extlinux_boot_files

* dtb_boot_files - converts dtb entries from KERNEL_DEVICETREE (assumed
  in DEPLOY/kernel) into IMAGE_BOOT_FILES ones (assuming destination is
  the root of the filesystem)
* extlinux_boot_files - converts extlinux files (assumed in
  DEPLOY/bootloader/extlinux) to IMAGE_BOOT_FILES entries (assuming
  destination is relative to the deploy directory of the extlinux files)

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>