nrf 52840 build nsib + mcuboot with public key only

[VIncentFIH]

Hi Noridc expert , 

I am trying to configure using nsib + mcuboot on nrf52840,

I need to build the image with only public key avaliable , and sign the image else where. 

I have seem exmaple here, it manual sign with mcuboot by replacing build\mcuboot\zephyr\autogen-pubkey.c

samples_for_nrf_connect_sdk/bootloader_samples/keys_and_signatures/mcuboot_manual_sign at main · hellesvik-nordic/samples_for_nrf_connect_sdk

but how about nsib ?