mtd-utils: flash_erase: Add an option for JFFS2 cleanmarker size

mtd-utils: flash_erase: Add an option for JFFS2 cleanmarker size

JFFS2 supports buffer mode for ECC'd NOR Flash that cleanmarker size
is rounded up to mtd->writesize, while the '-j' option in flash_erase
utility uses fixed 12 bytes. That makes flash program ops unaligned to
mtd->writesize and causes program error or disables ECC.

The mkfs.jffs2 utility supports '-c' option that allows users to specify
cleanmarker size. This patch implements '-c' option in the flash_erase
that can be used along with '-j' option.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>