mynewt: Add call to custom pre-boot function

Custom boot_preboot() function can be used to start
DFU in similar way as is already done in zephyr.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>
diff --git a/boot/mynewt/syscfg.yml b/boot/mynewt/syscfg.yml
index d472c20..8334444 100644
--- a/boot/mynewt/syscfg.yml
+++ b/boot/mynewt/syscfg.yml
@@ -28,6 +28,9 @@
     BOOT_CUSTOM_START:
         description: 'Override hal_system_start with a custom start routine'
         value: 0
+    BOOT_PREBOOT:
+        description: 'Call boot_preboot() function before booting application'
+        value:
 
 syscfg.vals:
     SYSINIT_CONSTRAIN_INIT: 0