mtd: add -c option for specifying amount of data to be used for checksum

So far fixtrx was calculating checksum over amount of data matching
partition erase size. It was mostly a workaround of checksum problem
after changing anything in initial TRX content (e.g. formatting JFFS2).
Its main purpose was to make bootloader accept modified TRX. This didn't
provide much protection of flash data against corruption.

This new option lets caller request calculating checksum over a bigger
amount of data. It may be used e.g. to include whole kernel data for
checksum and hopefully make bootloader go info failsafe mode if
something goes wrong.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>