Add `uefi` os constraint (#97)

diff --git a/os/BUILD b/os/BUILD
index 8e9f990..2ba8529 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -127,3 +127,8 @@
     name = "chromiumos",
     constraint_setting = ":os",
 )
+
+constraint_value(
+    name = "uefi",
+    constraint_setting = ":os",
+)