Add a method to re-use meta data from stx repo

Add a method to re-use meta data from stx repo

StarlingX has many repos to maintain meta data (patches and config files)
for packages, previously we added a local copy of those files as bitbake's
metadata for each recipe which may cause maintenance issue.

So add a method to avoid that, which includes:

* Add new recipes stx-${STX_REPO}-source for thoes stx git repo and put
  them into the work-shared directory so they can be used by other recipes
  that need the stx  meta data, so it will reduce downloads and increase
  re-use.

* Add stx-source.bbclass for easily creating the work-shared recipes

* Add stx-metadata.bbclass for those recipes that need the stx meta data,
  it will add dependency on the stx-${STX_REPO}-source and redefine the
  SRC_URI before do_patch, so it can find the patches in stx repo in
  work-shard and do the patch, and other meta data like config files can
  be used in do_install, then the local copy of those meta data can be
  removed, and for the future upgrade, we don't need to manually check
  and update those meta data any more.

Issue-ID: INF-215

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Change-Id: Ieb27b4886e3cd9ccb4a9cbf63085bf28dda29549