configs/at91sam9g20dfc: add ubifs target filesystem

This is a required component to complete the configuration. It also makes sense
to add ubifs so that this board can be flashed with the board/atmel/flasher.sh
script.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/at91sam9g20dfc_defconfig

@@ -15,6 +15,13 @@ BR2_LINUX_KERNEL_INTREE_DTS_NAME="at91sam9g20ek"
 
 # Filesystem
 BR2_TARGET_ROOTFS_TAR=y
+BR2_TARGET_ROOTFS_UBI=y
+BR2_TARGET_ROOTFS_UBI_PEBSIZE=0x20000
+BR2_TARGET_ROOTFS_UBI_SUBSIZE=512
+BR2_TARGET_ROOTFS_UBIFS=y
+BR2_TARGET_ROOTFS_UBIFS_LEBSIZE=0x1f800
+BR2_TARGET_ROOTFS_UBIFS_MINIOSIZE=0x800
+BR2_TARGET_ROOTFS_UBIFS_MAXLEBCNT=2047
 
 # Bootloaders
 BR2_TARGET_AT91BOOTSTRAP3=y