[arch-general] Check for update in script

[Yamakaky]

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