board/bsh/imx6ulz-bsh-smm-m2: flash U-Boot

Fix the board flashing by adding the bootloader, which I had mistakenly
forgotten to include in the script.

Fixes: 322e8d84 ("configs/imx6ulz_bsh_smm_m2_defconfig: new defconfig")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>