oe/copy_buildsystem.py: make sure layer exists

It had a problem when nested layer before, e.g.:
layer_a/layer_b/

And when layer_b is handled before layer_a, then layer_a dir existed, so
it would be treated as already handled, which was wrong, check
conf/layer.conf can fix the problem.

(From OE-Core rev: 2eaefa0c3ae589111266c7d6822428ad910415f4)

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

meta/lib/oe/copy_buildsystem.py

@@ -95,7 +95,7 @@ class BuildSystem(object):
                     destname = os.path.join(layerdestpath, f_basename)
                     _smart_copy(f, destname)
             else:
-                if os.path.exists(layerdestpath):
+                if os.path.exists(os.path.join(layerdestpath, 'conf/layer.conf')):
                     bb.note("Skipping layer %s, already handled" % layer)
                 else:
                     _smart_copy(layer, layerdestpath)