cmd: Rework disk.c usage

We only need the function found in cmd/disk.c when we have IDE, SCSI or
USB_STORAGE enabled.  While the first two are easy to get right, in the
3rd case we assume that the set of cases where we do have USB and do not
enable USB_STORAGE are small enough that we can take the small bloat of
un-discarded strings on gcc prior to 6.x

Signed-off-by: Tom Rini <trini@konsulko.com>