This commit cleans the configure.ac script and introduces following
changes:

- Rename --with-stage1-image flag to -with-stage1-default-location, no
  changes in meaning.

- Add --with-stage1-flavors option which take a comma-separated list
  of flavor names as a value. The specified flavors will be built. The
  default value depends on setup described below.

- Remove the --with-stage1 option in favor of different setup. When
  configuring a project, developer can use either a flavor setup or a
  detailed setup.

  The detailed setup has two flags - --with-stage1-default-name and
  --with-stage1-default-version. These can be used to specify some 3rd
  party stage1 image as a default. By default no stage1 flavors are
  built in this setup.

  The flavor setup has only one flag - --with-stage1-default-flavor.
  It takes a flavor name, which will become a default. The flavor must
  be a part of --with-stage1-flavors. Based on flavor name, stage1
  name and version are deduced (version is the same as rkt
  version). By default both coreos and kvm flavors are built in this
  setup and the first flavor in the list is the default.

- Require gpg only when either building coreos flavor or running
  functional tests.