nand_util: drop trailing all-0xff pages if requested

nand_util: drop trailing all-0xff pages if requested

Add a flag to nand_read_skip_bad() such that if true, any trailing
pages in an eraseblock whose contents are entirely 0xff will be
dropped.

The implementation is via a new drop_ffs() function which is
based on the function of the same name from the ubiformat
utility by Artem Bityutskiy.

This is as-per the reccomendations of the UBI FAQ [1]

[1] http://www.linux-mtd.infradead.org/doc/ubi.html#L_flasher_algo

Signed-off-by: Ben Gardiner <[email protected]>
CC: Artem Bityutskiy <[email protected]>
Acked-by: Detlev Zundel <[email protected]>
CC: Scott Wood <[email protected]>
Signed-off-by: Scott Wood <[email protected]>