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/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index be06271..f44ab02 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -75,6 +75,9 @@
 #if defined(MCUBOOT_RAM_LOAD)
     res |= BOOTUTIL_CAP_RAM_LOAD;
 #endif
+#if defined(MCUBOOT_DIRECT_XIP)
+    res |= BOOTUTIL_CAP_DIRECT_XIP;
+#endif
 
     return res;
 }