Prevent sysinstall from needlessly waiting for confirmation when using an USB

Prevent sysinstall from needlessly waiting for confirmation when using an USB
device in non-interactive mode.

If there are no USB devices, sysinstall gives an error messages, and if there
is >1, it'll ask which one is to be used. This change allows a non-interactive
install from USB media to succeed without any user interaction if there is
exactly one USB disk device in the system it can use.

Submitted by:	Daniel O'Connor < doconnorat gsoft dot com dot au >
Reviewed by:	randi
Approved by:	re (rwatson)