Reading package lists...
W: GPG error: http://archive.debian.org/debian jessie Release: The following signatures were invalid: KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717
W: The repository 'http://archive.debian.org/debian jessie Release' is not signed.
+ mkdir -p -m 777 /qemu/powerpc
+ cd /qemu/powerpc
+ apt-get -d --no-install-recommends download busybox:powerpc dropbear:powerpc libc6:powerpc libgcc1:powerpc libstdc++6:powerpc linux-image-4.9.0-0.bpo.6-powerpc:powerpc ncurses-base zlib1g:powerpc
WARNING: The following packages cannot be authenticated!
  dropbear:powerpc
E: Some packages could not be authenticated
The command '/bin/sh -c /linux-image.sh powerpc' returned a non-zero code: 100

PowerPC seems not to have long term support for powerpc:
https://www.debian.org/releases/jessie/

The problem is that the keys which signed the powerpc packages have
expired so they will always fail package verification. Disabling
signature verification will get around that but has obvious security
concerns.