ppc64el support for autopkgtest-build-qemu (#926945)

• Set kernel release name for ppc64el

  From: Thierry Fauck

  Partially addresses #926945.

  [smcv: Split out from a larger commit]

• autopkgtest-build-qemu: Generalize --efi into --boot=

  This will let us add --boot=ieee1275. While I'm here, add --boot=none for porting new architectures: in this case developers will have to set up the bootloader via --script, or directly boot a suitable kernel with autopkgtest-virt-qemu --qemu-options='-kernel ...'.

• autopkgtest-build-qemu: Add grub-ieee1275 support, default on ppc64el

  Very loosely based on a patch by Thierry Fauck.

  Co-authored-by: Thierry Fauck
  Thanks: Christian Kastner
  Closes: #926945