Add a capability for querying downgrade prevention

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index 0ca7b8c..c15ecb5 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -57,6 +57,9 @@
 #if defined(MCUBOOT_VALIDATE_PRIMARY_SLOT)
     res |= BOOTUTIL_CAP_VALIDATE_PRIMARY_SLOT;
 #endif
+#if defined(MCUBOOT_DOWNGRADE_PREVENTION)
+    res |= BOOTUTIL_CAP_DOWNGRADE_PREVENTION;
+#endif
 
     return res;
 }