sim: Remove more cfg around validate-slot-0

Turn some more conditional compilation into runtime decisions based on
how the code being tested is compiled.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/caps.rs b/sim/src/caps.rs
index dec879b..499add0 100644
--- a/sim/src/caps.rs
+++ b/sim/src/caps.rs
@@ -11,6 +11,7 @@
     OverwriteUpgrade = (1 << 4),
     EncRsa           = (1 << 5),
     EncKw            = (1 << 6),
+    ValidateSlot0    = (1 << 7),
 }
 
 impl Caps {