ubi: add percent evaluation for the bad blocks

ubi: add percent evaluation for the bad blocks

So far the plugin contains the evaluation of the maximum erase count
from `/sys/class/ubi/ubi<x>/max_ec` and the evaluation of bad physical
erase block count from `/sys/class/ubi/ubi<x>/bad_peb_count`.

For each ubi device, the reserved physical erase blocks are also displayed.
The value is shown in `/sys/class/ubi/ubi<x>/reserved_for_bad`.

If the defekt erase blocks are now used with the reserved erase blocks,
we can calculate who many percent of the flash is defekt.

With this information we can now create a notify that is triggered at a
certain percentage.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>