base-files: fwtool: Fix wrong checksum on combined-image with metadata

base-files: fwtool: Fix wrong checksum on combined-image with metadata

If I create following image:

define Device/engenius-m36
  IMAGE/sysupgrade.bin := combined-image | append-metadata
endef

Sysupgrade then errors out:

  Invalid image. Contents do not match checksum (image:cd285595eaf297370404ae0e2815ec1a calculated:2cf9a2286fb6b01af3ea189128017d44)
  Image check 'platform_check_image' failed.

By removing the metadata from the image I get combined-image checksum
working again and sysupgrade works.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(backported from 41770add03ad77a0ce41ed424ad050238f7d9272)