Let's find out the name of the device you have the kernel
on Open Firmware level. Type devalias first, and you may get
  hd0     ...
  hd1     ...
  ultra1  ...
  enet    ...
  cd      ...
etc. Then try one of them.
0 > boot enet:  hd0:
0 > boot enet:  ultra1:
0 > boot enet:  cd:
0 > boot enet:  ide0:

The second argument is the device to find out kernel.
Left to the colon is the name of the device.

I have iBook/SE, bootps server setup with /etc/bootptab,
        :hd=/:\
        :bf=ofwboot.elf:\
with /etc/inetd.conf setup as
 bootps dgram udp wait root /usr/sbin/bootpd    bootpd
 tftp   dgram udp wait root /usr/libexec/tftpd  tftpd -l -s /tftpboot
and /tftpboot/ofwboot.elf placed, and type
 0 > boot enet:0 cd:0
boots up from the CDROM kernel

Many Macintosh has problem typing any characters before
kernel actually load up, so avoid to interact as much as
possible once before you got it is ever OK.
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http://www.ki.nu/software/NetBSD/iBook2/