On disabling linux-libc-dev:
<plugwash> can anyone tell me how to modify the debian kernel packaging to disable linux-libc-dev?
<bwh> plugwash: Remove 'binary-arch-arch: install-libc-dev_$(ARCH)' from debian/rules.real

On disabling udebs that have become empty:
Delete the file from debian/installer/armhf/modules/armhf-rpi
Force the regeneration of debian/control (easiest way is to touch
debian/changelog and then run debian/rules debian/control-real