diff --git a/os/BUILD b/os/BUILD
index 599968e..2b3aeda 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -33,6 +33,12 @@
     constraint_setting = ":os",
 )
 
+# For platforms with no OS, like microcontrollers.
+constraint_value(
+    name = "none",
+    constraint_setting = ":os",
+)
+
 ### Apple OS Values
 
 constraint_value(