Clone nrfxlib next to the MCUboot root folder. So that it's located ../nrfxlib from MCUboot root folder.
make sure root-ec-p256.pem is set as the certificate and that CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256 is selected not CONFIG_BOOT_SIGNATURE_TYPE_RSA in prj.conf of boot/zephyr. Since it defaults to tinycrypt you'll have to go into menuconfig and change the implementation selection to cc310 or also set this in prj.conf.
mkdir build && cd build cmake -GNinja -DBOARD=nrf52840dk_nrf52840 ninja flash
Build a hello world example in zephyr and sign it with imgtool.py with the root-ec-p256.pem and flash it at FLASH_AREA_IMAGE_0.