Add Zephyr config for boostrap feature

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index eafdb94..77a695e 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -72,6 +72,14 @@
 	  swapping them.  This prevents the fallback recovery, but
 	  uses a much simpler code path.
 
+config BOOT_BOOTSTRAP
+	bool "Boostrap erased slot0 from slot1"
+	default n
+	help
+	  If y, enables bootstraping support. Bootstrapping allows an erased
+	  slot0 to be initialized from a valid image in slot1.
+	  If unsure, leave at the default value.
+
 config BOOT_ENCRYPT_RSA
 	bool "Support for encrypted upgrade images"
 	default n