Fix support for low memory devices (256 MB Pi)

By the default the Linux kernel limits the maximum size of the root tmpfs file system to half the size of usable memory.
This is too little on a 256 MB Pi, so resize it if necessary.

Closes #90
Add shut down button to boot menu

To safely power off the Pi

Closes #98
Update kernel config
Add support for installing images from CIFS network share

Closes #96