Mount /var as tmpfs and limit the use of tmpfs

This commit (apart from the cosmetic change to avoid using 'none') changes the use of ramfs into tmpfs. The reason for this is that ramfs usage can fill 100% of RAM and cannot be swapped out. While tmpfs can be swapped out and can be limited in size.

The size chosen is arbitrary, but taking into account the ATV1 hardware 5% of 256MB means 12MB and 20% means 50MB, considering we have swap this is not too drastic. For any larger system, these sizes are fine as well as we do not expect /var or /dev/shm to be filled completely. So worst case is covered, but not expected.