Linux_HAL_Essentials: Update README for Ubuntu.

Added a note pointing out the instructions for precise.
Storage: Change storage folder in Linux to `/var/APM`
ArduCopter: Change logs folder in Linux to `/var/APM/logs`
ArduCopter: support logging while disarmed

Patch applied taking using Andrew Tridgell's patch available at
https://github.com/tridge/ardupilot/commit/355581bf955a7d2d96da0e51c25625015b7e0216