The Linux/m68k kernel supports a "debug" option on the command line. There are several options for this on the Amiga and on Ataris:
debug=ser: Send debug output to the first serial device (usually the internal one on an Amiga; Modem1 on most Ataris; Modem2 on the Falcon). Your terminal should be set for 9600 bps, 8 data bits, 1 stop bit, no parity.
debug=mem: On Amigas (only), save the kernel output to a reserved block of chip memory. This output can be read after a reboot in AmigaOS by "dmesg," which is available at ftp://tsx-11.mit.edu/pub/linux/680x0/tools/amiga/dmesg.gz.
debug=midi: On Ataris (only), send output to the MIDI port (31250 bps, 8N1).
debug=par: On Ataris (only), send output to the printer port. Probably only useful with a line printer (i.e. dot matrix or daisy wheel).