feat(livenet): add memory size check depending on live image size

feat(livenet): add memory size check depending on live image size

For writeable live images, the memory must hold the complete image.
That means it must be at least the size of the image plus some RAM
necessary to run processes.
With too small RAM, the OOM Killer steps in and makes the boot hang.
This patch lets the system go into the emergency shell instead.

The default minimum RAM after subtracting the live image size is set to 1G.
The parameter rd.minmem is added to modify this.