Rework

General cleanup, refactored functions.
Checking if extroot is already running: if so,  sketch execution is halted, proceeds normally otherwise
SD card presence is done with /mnt/sda1 (so a properly formatted sd card is needed): other checks were unreliable
Added delay between fdisk and mkfs, to let the yun find the partitions.
It creates folders arduino/www on the data partition.
Reboot is just a comment, as the command does not work