This does what you want - download and read the documentation, it does far more than what the initial readme suggest.
http://aminet.net/package/util/boot/bootctrlWith this tool you can predefine your own defaults for the early startup menu, disable devices and tell what device to boot from.
For example, on my A600 that has no floppy drive and where I don't want it to look for DF0, I put this on the top of S:Startup-Sequence:
BootControl INSTALL BOOTDEV=DH0 HIDDEN=DF0 QUIET