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/include/bootutil/caps.h b/boot/bootutil/include/bootutil/caps.h
index 553fa5d..3205fa2 100644
--- a/boot/bootutil/include/bootutil/caps.h
+++ b/boot/bootutil/include/bootutil/caps.h
@@ -44,6 +44,7 @@
 #define BOOTUTIL_CAP_ED25519                (1<<9)
 #define BOOTUTIL_CAP_ENC_EC256              (1<<10)
 #define BOOTUTIL_CAP_SWAP_USING_MOVE        (1<<11)
+#define BOOTUTIL_CAP_DOWNGRADE_PREVENTION   (1<<12)
 
 /*
  * Query the number of images this bootloader is configured for.  This
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;
 }