we provide Secure Boot application note including demo projects which explains all the steps in detail. It can be downloaded from:
https://www.nxp.com/products/processors-and-microcontrollers/s32-automotive-platform/s32k-auto-gener...
Application note can be found here:
Documentation -> Secure Files -> Secure Boot Application note v0.1.1.0 (AN744511)
Associated demo project can be downloaded here:
Design Resources -> Software -> Secure Files -> SecureBootAppNoteDemo (SW745310)
Pay attention to mainly to sections 7 and 8.
Hope it helps