Add Zephyr config for boostrap feature

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index b6bf527..8a88abc 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -9,6 +9,8 @@
 
 CONFIG_BOOT_ENCRYPT_RSA=n
 
+CONFIG_BOOT_BOOTSTRAP=n
+
 ### Default to RSA
 CONFIG_BOOT_SIGNATURE_TYPE_RSA=y
 CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256=n