dfu: Add option to skip empty pages when flashing UBI images to NAND

Add a new option to enable the DROP_FFS flag when flashing UBI images to
NAND in order to drop trailing all-0xff pages.

This is similar to the existing FASTBOOT_FLASH_NAND_TRIMFFS option.

Signed-off-by: Guillermo Rodriguez <guille.rodriguez@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>