iquik: cleanup

Make this a bit more maintainable.

Signed-off-by: Andrei E. Warkentin <andrey.warkentin@gmail.com>
iquik: preboot support

Done better. Now the OF script can follow the regular boot
block (no need to chain).

My example for a boot floppy:

" ata0/ata-disk@0:4" " boot-file" $setenv
eject fd

Signed-off-by: Andrei E. Warkentin <andrey.warkentin@gmail.com>