make-initrd: various improvements

- 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>

pkgs/build-support/kernel/make-initrd.sh

@@ -40,9 +40,12 @@ for PREP in $prepend; do
   cat $PREP >> $out/initrd
 done
 (cd root && find * .[^.*] -exec touch -h -d '@1' '{}' +)
-(cd root && find * .[^.*] -print0 | sort -z | cpio -o -H newc -R +0:+0 --reproducible --null | $compressor >> $out/initrd)
+(cd root && find * .[^.*] -print0 | sort -z | cpio -o -H newc -R +0:+0 --reproducible --null | eval -- $compress >> "$out/initrd")
 
 if [ -n "$makeUInitrd" ]; then
-    mv $out/initrd $out/initrd.gz
-    mkimage -A $uInitrdArch -O linux -T ramdisk -C gzip -d $out/initrd.gz $out/initrd
+    mkimage -A $uInitrdArch -O linux -T ramdisk -C "$uInitrdCompression" -d $out/initrd"$extension" $out/initrd.img
+    # Compatibility symlink
+    ln -s "initrd.img" "$out/initrd"
+else
+    ln -s "initrd" "$out/initrd$extension"
 fi