sysupgrade: do not require an extra argument after --restore-backup

Neither --create-backup nor --restore-backup need an image name (following
the backup filename). Treat them in uniform way.

Mostly fixes #12346.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt@34974 3c298f89-4303-0410-b956-a3cf2f4a3e73
sysupgrade: clarify online help

Fixes #12346.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt@34975 3c298f89-4303-0410-b956-a3cf2f4a3e73