[d-i] build fail to report a missing dpkg-architecture

[Petter Reinholdtsen <petter.reinholdtsen@usit.uio.no>]

When trying to build a d-i floppy without dpkg-architecture from the
dpkg-dev package, this error message appear:

  Reading Package Lists...
  Building Dependency Tree...
  E: Couldn't find package kernel-image---udeb

It would be better if the missing architecture information was
reported.  This is the failing make code:

  architecture    := $(shell dpkg-architecture -qDEB_HOST_ARCH)

  # The version of the kernel to use.

  ifeq "$(architecture)" "i386"
  KVERS=2.4.19
  FLAVOUR=386
  KERNELNAME=vmlinuz
  endif
  ifeq "$(architecture)" "powerpc"
  KVERS=2.4.19
  FLAVOUR=powerpc
  KERNELNAME=vmlinux
  endif

There should be some 'else fatal_error(Call to dpkg-architecture
failed)' or something similar to detect this situation.  Any ideas how
this should be done?