bookworm kernel headers for camera driver kernel module

Start with the reported Kbuild and the external-module command using the 6.6.31 Bookworm headers, then compare it with the successful 6.1.21 build. Reproduce the missing imx219-overlay.dtbo target and inspect the kernel-header build behavior around the listed Kbuild entries. Done means the imx219 module and device-tree overlay build successfully against the Bookworm headers.

LINK TO FORUM:
https://forums.raspberrypi.com/viewtopic.php?p=2248093#p2248093

for a custom camera driver I'm trying to build a dkms module.
For sake of simplicity, let's reproduce it with the imx219 driver.
basically I have a folder structure like this:

    pi@raspberrypi:~/ams_rpi_kernel/imx219/src $ tree
    .
    ├── imx219.c
    ├── imx219.dtsi
    ├── imx219-overlay.dts
    ├── Kbuild
    └── Makefile 

pi@raspberrypi:~/ams_rpi_kernel/imx219/src $ make -C /lib/modules/6.6.31+rpt-rpi-v8/build  M=$PWD CPATH=/usr/src/linux-headers-6.6.31+rpt-common-rpi/include/ 
make: Entering directory '/usr/src/linux-headers-6.6.31+rpt-rpi-v8'
  CC [M]  /home/pi/ams_rpi_kernel/imx219/src/imx219.o
make[2]: *** No rule to make target '/home/pi/ams_rpi_kernel/imx219/src/imx219-overlay.dtbo', needed by '/home/pi/ams_rpi_kernel/imx219/src/'.  Stop.
make[1]: *** [/usr/src/linux-headers-6.6.31+rpt-common-rpi/Makefile:1938: /home/pi/ams_rpi_kernel/imx219/src] Error 2
make: *** [/usr/src/linux-headers-6.6.31+rpt-common-rpi/Makefile:246: __sub-make] Error 2
make: Leaving directory '/usr/src/linux-headers-6.6.31+rpt-rpi-v8'

on older kernel versions, this command worked fine for me. (6.1.y headers + imx219.c from that kernel)

pi@raspberrypi:~/ams_rpi_kernel/imx219/src61 $ make -C /lib/modules/6.1.21-v8+/build/  M=$PWD CPATH=/usr/src/linux-headers-6.1.21-v8+/include/
make: Entering directory '/usr/src/linux-headers-6.1.21-v8+'
warning: the compiler differs from the one used to build the kernel
  The kernel was built by: gcc (Debian 10.2.1-6) 10.2.1 20210110
  You are using:           gcc (Debian 12.2.0-14) 12.2.0
  CC [M]  /home/pi/ams_rpi_kernel/imx219/src61/imx219.o
  DTCO    /home/pi/ams_rpi_kernel/imx219/src61/imx219.dtbo
  MODPOST /home/pi/ams_rpi_kernel/imx219/src61/Module.symvers
  CC [M]  /home/pi/ams_rpi_kernel/imx219/src61/imx219.mod.o
  LD [M]  /home/pi/ams_rpi_kernel/imx219/src61/imx219.ko
make: Leaving directory '/usr/src/linux-headers-6.1.21-v8+'

more info:
I'm using pi4b, 64bit, bookworm. kernel 6.6.31.

pi@raspberrypi:~/ams_rpi_kernel/imx219/src $ uname -a
Linux raspberrypi 6.6.31+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.6.31-1+rpt1 (2024-05-29) aarch64 GNU/Linux

contents of the files:
Kbuild

obj-m  := imx219.o
dtbo-y += imx219.dtbo
targets += $(dtbo-y)
always  := $(dtbo-y)
always-y        := $(dtbo-y)