boot: bootutil add cap for slot 0 validation

Add a capability to query if MCUboot has been built to verify slot 0.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/bootutil/src/caps.c b/boot/bootutil/src/caps.c
index c341f8b..85bd6fd 100644
--- a/boot/bootutil/src/caps.c
+++ b/boot/bootutil/src/caps.c
@@ -41,6 +41,9 @@
 #if defined(MCUBOOT_ENCRYPT_KW)
 	res |= BOOTUTIL_CAP_ENC_KW;
 #endif
+#if defined(MCUBOOT_VALIDATE_SLOT0)
+	res |= BOOTUTIL_CAP_VALIDATE_SLOT0;
+#endif
 
         return res;
 }