This will allow in the future to use a different initrd option when
building images, for example dracut.
Also drop old make-inird support.

This one has filled up the entire 8 Mb tmpfs
while running inside installer stage1 on a
124-drive system (configured as JBOD).

Reported-by: http://norsi-trans.ru

Kernel packages we have on this architecture use
uncompressed vmlinux file called image; handle that.

stage2 has been thinking it's synonymous with propagator
and used to usurp kernel's belongings either; carefully
tear scripts apart so that kernel feature makes sure
initrd gets generated, and stage2 (which is still all
about propagator) cares for its bits.