base-files: always use staged sysupgrade

Support for the -d and -p options is dropped; it may be added again at some
point by adding these flags to the ubus sysupgrade call.

A downside of this is that we get a lot less information about the progress
of the upgrade: as soon as the actual upgrade starts, all shell sessions
are killed to allow unmounting the root filesystem.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>