build_kernel_image: Enable kernel image build for arm64

BUG=None
TEST='Run build_image script'

Change-Id: I45009a6e9dad6fff03d8207e1377a8ffd72061bb
Reviewed-on: https://chromium-review.googlesource.com/1127990
Commit-Ready: Adam Kallai <kadam@inf.u-szeged.hu>
Tested-by: Adam Kallai <kadam@inf.u-szeged.hu>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed