[arch-general] Check for update in script

Hi I can't find a good way to check for available upgrades without touching the database, as running `pacman -Sy` without `u` isn't supported. Any idea ? Maybe a new pacman switch like "--upstreamdbpath" to toggle change the location of sync/ without touching local/ ? Why I need it : https://github.com/XavierBerger/RPi-Monitor/pull/57#issuecomment-55248969 Yamakaky