oe/copy_buildsystem.py: dereference symlink

When there is a relative symlink in the layer, for example:
symA -> ../out/of/layer/file

symA will be invalid fater copied, it would be invalid from build time
if it points to a relative path, and would be invalid after extracted
the sdk if it points to a absolute py. Dereference symlink when copy
will fix the problem.

Use tar rather than shutil.copytree() to copy is because:
1) shutil.copytree(symlinks=Fasle) has bugs when dereference symlinks:
   https://bugs.python.org/issue21697
   And Ubunutu 1404 doesn't upgrade python3 to fix the problem.

2) shutil.copytree(symlinks=False) raises errors when there is a invalid
   symlink, and tar just prints a warning, tar is preferred here since
   the real world is unpredicatable

3) tar is faster than shutil.copytree() as said by oe.path.copytree()

So use tar to copy.

(From OE-Core rev: f4d70bb0882eec4fb46cd942f2796fad57c72982)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/copy_buildsystem.py

@@ -4,11 +4,15 @@ import stat
 import shutil
 
 def _smart_copy(src, dest):
+    import subprocess
     # smart_copy will choose the correct function depending on whether the
     # source is a file or a directory.
     mode = os.stat(src).st_mode
     if stat.S_ISDIR(mode):
-        shutil.copytree(src, dest, symlinks=True, ignore=shutil.ignore_patterns('.git'))
+        bb.utils.mkdirhier(dest)
+        cmd = "tar --exclude='.git' --xattrs --xattrs-include='*' -chf - -C %s -p . \
+        | tar --xattrs --xattrs-include='*' -xf - -C %s" % (src, dest)
+        subprocess.check_output(cmd, shell=True, stderr=subprocess.STDOUT)
     else:
         shutil.copyfile(src, dest)
         shutil.copymode(src, dest)