bootstd: Drop the system bootdev

This was a work-around for the fact that global bootmeths such as EFI
bootmgr and VBE don't use a particular bootdev, or at least select it
themselves so that we don't need to scan all bootdevs when using that
bootmeth.

Drop the system bootdev entirely.

Signed-off-by: Simon Glass <sjg@chromium.org>