sim: Support direct-xip configuration

Add simulator support for building the direct-xip configuration.
Although this builds, there are no tests that test any of the
functionality, so all current tests trivially pass.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/Cargo.toml b/sim/mcuboot-sys/Cargo.toml
index 42daf2d..ffbce48 100644
--- a/sim/mcuboot-sys/Cargo.toml
+++ b/sim/mcuboot-sys/Cargo.toml
@@ -72,6 +72,11 @@
 # image is copied to RAM before loading it.
 ram-load = []
 
+# Support simulation of direct XIP.  No swaps are performed, the image
+# is directly executed out of whichever partition contains the most
+# appropriate image.
+direct-xip = []
+
 # Check (in software) against version downgrades.
 downgrade-prevention = []