devserver: use emaint-$BOARD

The /usr/lib/portage/bin/emaint path no longer exists in recent portage
versions.  But rather than use the updated internal path, lets switch to
the existing emaint-$BOARD wrapper.

BUG=chromium:462099
TEST=`cbuildbot amd64-generic-full` passes w/new & old portage

Change-Id: Ia1ced6fdc4d5ef1cd6b6b9eee2bfc27cdf6f643f
Reviewed-on: https://chromium-review.googlesource.com/254681
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/builder.py b/builder.py
index c24422a..51dda9c 100644
--- a/builder.py
+++ b/builder.py
@@ -159,9 +159,7 @@
   if changed:
     env_copy = os.environ.copy()
     env_copy['PKGDIR'] = gmerge_pkgdir
-    env_copy['ROOT'] = root
-    env_copy['PORTAGE_CONFIGROOT'] = root
-    cmd = ['/usr/lib/portage/bin/emaint', '-f', 'binhost']
+    cmd = ['emaint-%s' % board, '-f', 'binhost']
     subprocess.check_call(cmd, env=env_copy)
 
   return bool(installed_matches)