mtd: add logic for TP-Link ramips recovery magic

mtd: add logic for TP-Link ramips recovery magic

This adds an option to set the recovery flag of newer TP-Link MediaTek
boards and remove it after a successful write.

To make use of this feature, add the '-t' option to mtd-write.

The '-t' option takes the mtd partition containing the recovery flag
(usually 'romfile') as an argument. Make sure this partition is not
flagged as read-only!

Example:
 > mtd -t romfile write owrt.bin firmware

This command writes the recovery-flag before it begins writing the image
to the firmware partition. After the image-write has been successful,
the recovery flag is removed.

This way, the TP-Link web-recovery is automatically enabled on an
unsucessful flash (e.g. power loss).

This option is only available if the mtd package is compiled for the
ramips target.

Signed-off-by: David Bauer <mail@david-bauer.net>