mips for example might use uImages too

IA64 (Itanium) is something completely different and certainly not
what we want! x86_64 code lives in arch/x86 just like "classic" x86.

- Generate a link to the initramfs file with an appropriate file
  extension, guessed based on the compressor by default
- Use correct metadata in u-boot images if generated, up to now this
  was hardcoded to gzip and would silently generate an erroneous image
  if another compressor was specified
- Document all the parameters
- Improve cross-building compatibility, by allowing passing either a
  string as before, or a function taking a package set and returning the
  path to a compressor in the "compressor" argument of the
  function.
- Support more compression algorithms
- Place compressor executable function and arguments in passthru, for
  reuse when appending initramfses

Co-Authored-By: Dominik Xaver Hörl <hoe.dom@gmx.de>

lz4 compression is excluded because it doesn't work for a reason which
remains unclear to me.

Initrd improvements