bootstrap image: build needed source packages and add them

* IsPackageAvailable, FetchPackage: Add flags parameter. The only
  flag supported ATM is nameResolved, indicating that the specified
  package name does not need to be resolved with respect to a secondary
  architecture anymore.
* Add build profile action "build-package-list". As an argument the
  file to which the list of all packages needed for the image is
  written. The rule BuildHaikuImagePackageList implements the action.
* BuildBootstrapRepositoryConfig: Does now require the variable
  HAIKU_REPOSITORY_TREE_PATH to be set on the config file target instead
  of hard-coding the path. Allows reuse of the actions.
* Add rules BuildHaikuPortsSourcePackageDirectory and
  BuildHaikuPortsRepositoryConfig. The former builds all HaikuPorts
  source packages needed to build the packages for an alpha image. The
  latter generates a haikuports.conf file for use on the bootstrap
  Haiku.
* HaikuImageBootstrap: Add directory /boot/home/haikuports which
  contains a subdirectory with the source packages and a
  haikuports.conf.