config: skip missing profiles/repo_name warning when using layout.conf:repo-name

The repo name is considered missing if there is no explicit "repo_name"
file.  But if it's set in layout.conf via repo-name, we can skip that.
Otherwise we end up with copy & paste spew.

BUG=b:20895978

Change-Id: I0ef4ff773e1d666b1e9d1b06b1191ce6b3755806
diff --git a/usr/lib/portage/pym/portage/repository/config.py b/usr/lib/portage/pym/portage/repository/config.py
index 5e0d055..24c3112 100644
--- a/usr/lib/portage/pym/portage/repository/config.py
+++ b/usr/lib/portage/pym/portage/repository/config.py
@@ -233,6 +233,7 @@
 				# useful when having two copies of the same repo enabled
 				# to avoid modifying profiles/repo_name in one of them
 				self.name = layout_data['repo-name']
+				self.missing_repo_name = False
 
 			for value in ('allow-missing-manifest',
 				'allow-provide-virtual', 'cache-formats',