Add checks that the board and variant names don't contain underscores.

bin/cros_overlay_list

Index: bin/cros_overlay_list
diff --git a/bin/cros_overlay_list b/bin/cros_overlay_list
index 5aeb2d49cea3e4865dc35a91f714d23cdf327e6d..1d768663e922cab082796f51466a27da862ad618 100755
--- a/bin/cros_overlay_list
+++ b/bin/cros_overlay_list
@@ -30,6 +30,16 @@ if [ -z "$FLAGS_board" ] ; then
   exit 1
 fi
 
+if [[ "$FLAGS_board" =~ "_" ]] ; then
+  error "--board name must not contain an underscore (_)."
+  exit 1
+fi
+
+if [[ "$FLAGS_variant" =~ "_" ]] ; then
+  error "--variant name must not contain an underscore (_)."
+  exit 1
+fi
+
 #
 # Check for chromeos-overlay.
 #