keymaster: use enum for boot color

Instead of an int, use an enum for boot color.

Change-Id: I4ab37d80218846bf37016c36c3f8bec724349981
diff --git a/nugget/proto/nugget/app/keymaster/keymaster.proto b/nugget/proto/nugget/app/keymaster/keymaster.proto
index fd35c4b..3344210 100644
--- a/nugget/proto/nugget/app/keymaster/keymaster.proto
+++ b/nugget/proto/nugget/app/keymaster/keymaster.proto
@@ -307,7 +307,7 @@
 message SetBootStateRequest {
   bool is_unlocked = 1;
   bytes public_key = 2;              // This is a SHA256 digest.
-  uint32 color = 3;
+  BootColor color = 3;
   uint32 system_version = 4;
   uint32 system_security_level = 5;
   bytes boot_hash = 6;               // This is a SHA256 digest.