src_distribute*.bbclass: cleanup, revamp parts, distribute all sources.

src_distribute*.bbclass: cleanup, revamp parts, distribute all sources.

- Kill the big shell function with the case statement used in _local in
  favor of a set of bitbake variables, with the appropriate one selected
  as SRC_DISTRIBUTECOMMAND.
- Add and use SRC_DISTRIBUTE_DLONLY to toggle distribution of all sources
  versus just those downloaded to DL_DIR.  This variable expects to be set
  to an integer, anything but 0 will enable dlonly operation.  This is
  disabled by default.
- Add a more sanely named distribute_sources_all, and make distsrcall
  depend on it, for compatibility.
- By default, distributes the sources into
  ${SRC_DISTRIBUTEDIR}/${LIC}/${PN}/.
- Move some _local specific bits, like use of SRC_DISTRIBUTEDIR, out of
  the main class.
- Set a "LIC" var along with the "SRC" when running SRC_DISTRIBUTECOMMAND,
  to facilitate the above.

Signed-off-by: Chris Larson <clarson@mvista.com>