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>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 9709b62..174fb58 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -42,6 +42,13 @@
 	  This option enables using DFU to read and write to NAND based
 	  storage.
 
+config DFU_NAND_TRIMFFS
+	bool "Skip empty pages when flashing UBI images to NAND"
+	depends on DFU_NAND
+	help
+	  When flashing UBI images to NAND, enable the DROP_FFS flag to drop
+	  trailing all-0xff pages.
+
 config DFU_RAM
 	bool "RAM back end for DFU"
 	help