For WP76xx, I remember if i modify the file /kernel/arch/arm/boot/dts/qcom/mdm9607-wp76xx.dtsi, after that i type “make” command, the yocto image will be including my modification.
I am not quite understand what “boot” package you are referring to…
For secure boot, you can also see this document.
I tried it in WP76 FW R16 before: