MCUboot is a secure bootloader for 32-bit MCUs.
The nRF Connect SDK includes a fork of the MCUboot project.
This fork is kept as close to the original repository as possible, but it might contain some additions that are specific to Nordic Semiconductor devices and applications.
You can find the fork in
bootloader/mcuboot after obtaining the nRF Connect SDK source code.
See the documentation of Nordic Semiconductor’s MCUboot fork for more information about MCUboot.
As with Zephyr RTOS, the downstream nRF Connect SDK MCUboot repository history is periodically rewritten to clean up intermediate work, merges of upstream, etc. See OSS repositories downstream project history for more details.