shill: update Makefile to enable -Wall, and related flags, on ebuilds

Previously, ebuild flags would be used in preference to those in
the Makefile. Now, we append -Wall, and related flags, to those
set by emake.

Also: update existing code to eliminate warnings.

BUG=chromium-os:15877
TEST=emerge

Change-Id: I95f80860edb29e197aef0e08493f78276da3c062
Reviewed-on: http://gerrit.chromium.org/gerrit/1742
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/device_info.cc b/device_info.cc
index 1495104..0172e81 100644
--- a/device_info.cc
+++ b/device_info.cc
@@ -127,7 +127,7 @@
   int rta_bytes;
   char *link_name = NULL;
   DeviceRefPtr device;
-  Device::Technology technology;
+  Device::Technology technology = Device::kUnknown;
   bool is_stub = false;
 
   VLOG(2) << __func__;