i.MX6UL: isiot: Add modeboot env via board_late_init

Add runtime, modeboot env which is setting mmcboot, or
nandboot based on the bootdevice so-that conditional
macros b/w MMC and NAND for CONFIG_BOOTCOMMAND should
be avoided in config files.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/configs/imx6ul_isiot_mmc_defconfig b/configs/imx6ul_isiot_mmc_defconfig
index 8ecdd8e..ea2d378 100644
--- a/configs/imx6ul_isiot_mmc_defconfig
+++ b/configs/imx6ul_isiot_mmc_defconfig
@@ -39,3 +39,4 @@
 CONFIG_PINCTRL_IMX6=y
 CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
+CONFIG_BOARD_LATE_INIT=y