Determine HAIKU_CPU and HAIKU_PACKAGING_ARCH in configure

... instead of in BuildSetup.
rule ExtractBuildFeatureArchives: implement placeholders

Add support for placeholders in the attribute values. The values of the
currently supported placeholders depend on the package file name
(package version, actual package and port name, etc.).
BuildFeature: get rid of hard-coded package versions

... using the new placeholder feature of ExtractBuildFeatureArchives.
Include DefaultBuildProfiles earlier

DefaultBuildProfiles is now included earlier (right after BuildSetup).
This allows us to set HAIKU_BOOTSTRAP_BUILD earlier, so it can be used
for the repository selection. The actual build profile definitions,
which cannot be done that early, live in the rule
DefineDefaultBuildProfiles, which is invoked where the file was
previously included.
Remove more stuff from the Haiku bootstrap packages/image

Namely GL/Mesa and libtiff.
Integrate building the HaikuPorts bootstrap packages

* Add configure option --bootstrap which allows specifying the
  haikuporter and HaikuPorts repository paths.
* Add rules for supporting a second repository type. The
  PackageRepository rule is now private and RemotePackageRepository is
  used for remote repositories. The new BootstrapPackageRepository rule
  is for defining a bootstrap repository (there will probably be only
  the HaikuPorts cross repository) whose packages can be built as needed
  via haikuporter.
* Rename DownloadPackage to FetchPackage.
* Define repository HaikuPortsCross.
* HaikuCrossDevel package(s): There are now two sets of packages: A
  "stage1" set with the same content as before and a final set
  additionally containing the libraries libbe, libnetwork, libpackage.
  Those are needed for building the libsolv bootstrap package while for
  building them we need other bootstrap packages (ICU, libz).

This is basically all that's required to build a bootstrap Haiku
completely from sources, with a few caveats:
* There's no ICU bootstrap recipe yet (so one has to cheat and use the
  prebuilt package ATM).
* Probably doesn't work on Haiku yet (tested on Linux only).
* A 32 bit environment must be used (otherwise building the gcc 2
  bootstrap package fails).
* Building with multiple jobs doesn't work yet, since haikuporter uses
  common directories for building different packages and there's no
  explicit serialization yet.
* Haven't tested the resulting image save for booting it. So it probably
  needs a bit more work before it can actually build the final
  HaikuPorts packages.