dfu: mtd: skip empty pages when writing page for UBI partition

Align the DFU MTD backend for the UBI partitions with the mtd command write
behavior when the option .dontskipff is not used: don't write the empty
pages (full of 0xFF); it is not required for UBI, see [1] for details.

This patch avoids the "free space fixup" procedure in the kernel [2]
and allows to program a UBIFS volume generated by mkfs.ubifs without the
option -F, --space-fixup.

The MTD DFU backend implements this behavior introduced on DFU NAND
backend by the commit 13cb7cc9 ("dfu: Add option to skip empty pages
when flashing UBI images to NAND") and also supported by the command nand
by CONFIG_CMD_NAND_TRIMFFS and by commit c9494866 ("cmd_nand: add nand
write.trimffs command").

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



Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>