cmd: mtd: check if a block has to be skipped or erased

As reported by patch [1], the `mtd erase' command should not erase bad
blocks.
To force bad block erasing you have to use the `mtd erase.dontskipbad'
command.

This patch tries to fix the same issue without modifying code taken
from the linux kernel, in order to make further upgrades easier.

[1] https://lore.kernel.org/all/20221006031501.110290-2-mikhail.kshevetskiy@iopsys.eu/


Suggested-by: Michael Trimarchi <michael@amarulasolutions.com>
Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Co-developed-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Tested-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>