Apply DTB patch to Image Builder

Hello, I recently replaced the SPI NOR Flash from my D-Link DIR-882 A1 from its original 16 MB with a 32 MB one.

I've cloned the OpenWrt repository locally, made small adjustments to both the DTS and make file and compiled the firmware locally, and, as expected, everything went well (I can successfully use the extra space via that custom built firmware). Since the changes were minimal, I'm trying to keep using the Image Builder instead of cloning the entire OpenWrt repository locally every time I want to update the router to a more recent snapshot.

I'm able to patch the necessary change to support the bigger flash memory directly in the DTB that ships with the Image Builder using fdtput, then it's just a matter of adjusting target/linux/ramips/image/mt7621.mk to produce a bigger firmware. However, the Image Builder seems to completely ignore any change made to the DTB, as I'm assuming it simply uses the pre-compiled dlink_dir-882-a1-kernel.bin instead.

If I didn't overlook anything, everything required to assemble the kernel binary ships with the Image Builder already (compiled vmlinux image and initramfs, as well as the DTB). So, how do I reassemble the kernel binary with the patched DTB? That's the only missing part of the puzzle...