Update disk space information

:Release Notes:
Update calculation and information regarding minimal free disk space in upstart script.

:Detailed Notes:
Remove hardcoded value of minimum required free space, and make it to calculate dinamically.

:Testing Performed:
Tested on local

:QA Notes:

:Issues Addressed:
[GF-53734] DB8 needs to force a factory reset if it has run out of space

Open-webOS-DCO-1.0-Signed-off-by: Bogdan Artemenko <bogdan.artemenko@lge.com>

Change-Id: Ib67d58e55e87cbf3bb3441eb3dcfa5a0bd06a65d
Reviewed-on: https://g2g.palm.com/4316
Reviewed-by: DCO Verification
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Oleksii Uratkin <oleksii.uratkin@lge.com>