Comment 5 for bug 1723203

Debian seems to be doing stuff through hooks in the raspi3-firmware package. They copy kernels to /boot/firmware this way and generate config.txt and cmdline.txt files through this mechanism. That's a bit rubbish if you want to change the defaults, plus you just end up with duplicate copies of all the kernels.

Also, if I'm reading this correctly https://anonscm.debian.org/cgit/d-i/libdebian-installer.git/tree/src/system/subarch-arm-linux.c , the addition of new subarches in armhf is discouraged.