fastboot: Migrate FASTBOOT_FLASH_NAND_TRIMFFS to Kconfig

Add FASTBOOT_FLASH_NAND_TRIMFFS to Kconfig; note there are no in-tree
users of it.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/fastboot/Kconfig b/drivers/fastboot/Kconfig
index 9bbc5c6..1d7caaf 100644
--- a/drivers/fastboot/Kconfig
+++ b/drivers/fastboot/Kconfig
@@ -86,6 +86,13 @@
 	  regarding the non-volatile storage device. Define this to
 	  the eMMC device that fastboot should use to store the image.
 
+config FASTBOOT_FLASH_NAND_TRIMFFS
+	bool "Skip empty pages when flashing NAND"
+	depends on FASTBOOT_FLASH_NAND
+	help
+	  When flashing NAND enable the DROP_FFS flag to drop trailing all-0xff
+	  pages.
+
 config FASTBOOT_GPT_NAME
 	string "Target name for updating GPT"
 	depends on FASTBOOT_FLASH_MMC && EFI_PARTITION