debian: Remove kernel prerm script

We remove the kernel prerm script because we don't ever want to leave
the system without a bootable kernel.

Signed-off-by: Sean Cross <xobs@kosagi.com>
novena: Change config to add hdmi, lcd hooks

Add hooks to call out to uenv.txt to configure HDMI, LCD, and other bits.

Signed-off-by: Sean Cross <xobs@kosagi.com>