nitrogen6x: u-boot: use imx format to match bootscript

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

configs/nitrogen6x_defconfig

@@ -14,6 +14,7 @@ BR2_TARGET_ROOTFS_EXT2_2r1=y
 # bootloader
 BR2_TARGET_UBOOT=y
 BR2_TARGET_UBOOT_BOARDNAME="nitrogen6q"
+BR2_TARGET_UBOOT_FORMAT_IMX=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/u-boot-imx6/tarball/4034391/u-boot-4034391.tar.gz"