Use the TARGET_* variable instead of the HAIKU_* one.

HAIKU_* variables are copied to TARGET_*, making it possible to also
build for other targets (libbe_build, or BeOS R5 when we still allowed
that).

Thanks to Ingo for explaining how this is meant to work.
Fix creation of haikuports.conf for bootstrap image.

* comments in here-documents don't work (the shell filters them out),
  so we echo the comment line explicitly