deb-pkg: Inhibit initramfs builders if CONFIG_BLK_DEV_INITRD is not set

The kernel postinst hook for initramfs-tools will build an initramfs
on installation unless $INITRD is set to 'No'.  make-kpkg generates a
postinst script that sets this variable appropriately, but we don't.
Set it based on CONFIG_BLK_DEV_INITRD.

This should also work with dracut when <http://bugs.debian.org/729622

>
is fixed.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>