Add capability for DIRECT_XIP feature

Add capability detection for the MCUBOOT_DIRECT_XIP feature.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/caps.rs b/sim/src/caps.rs
index 932b5ab..441494d 100644
--- a/sim/src/caps.rs
+++ b/sim/src/caps.rs
@@ -27,6 +27,7 @@
     Bootstrap            = (1 << 14),
     Aes256               = (1 << 15),
     RamLoad              = (1 << 16),
+    DirectXip            = (1 << 17),
 }
 
 impl Caps {
@@ -44,7 +45,7 @@
     /// Query if this configuration performs some kind of upgrade by writing to flash.
     pub fn modifies_flash() -> bool {
         // All other configurations perform upgrades by writing to flash.
-        !Self::RamLoad.present()
+        !(Self::RamLoad.present() || Self::DirectXip.present())
     }
 }