Support SYNC setting in make.conf.

This fixes commit 73188d23c68989539a913716bb8a4132d2c43316 so that it
works with make.conf.
diff --git a/pym/portage/package/ebuild/config.py b/pym/portage/package/ebuild/config.py
index 619d072..070a522 100644
--- a/pym/portage/package/ebuild/config.py
+++ b/pym/portage/package/ebuild/config.py
@@ -491,6 +491,7 @@
 			known_repos = []
 			portdir = ""
 			portdir_overlay = ""
+			portdir_sync = None
 			for confs in [make_globals, make_conf, self.configdict["env"]]:
 				v = confs.get("PORTDIR")
 				if v is not None:
@@ -500,9 +501,15 @@
 				if v is not None:
 					portdir_overlay = v
 					known_repos.extend(shlex_split(v))
+				v = confs.get("SYNC")
+				if v is not None:
+					portdir_sync = v
+
 			known_repos = frozenset(known_repos)
 			self["PORTDIR"] = portdir
 			self["PORTDIR_OVERLAY"] = portdir_overlay
+			if portdir_sync:
+				self["SYNC"] = portdir_sync
 			self.lookuplist = [self.configdict["env"]]
 			if repositories is None:
 				self.repositories = load_repository_config(self)