Hi, hyoungkikim
You can download the u-boot source from uboot-imx - i.MX U-Boot link in which you can find the "mx6ull_14x14_evk_qspi1_defconfig" config.
You can also download the u-boot source using the yocto build system as NXP use the yocto project envi ronment for the i.MX boards.
Please download the L4.9.11_1.0.0 document as shown below from i.MX Software and Development Tools | NXP link.

In this BSP documentation, you will find the "i.MX_Yocto_Project_User's_Guide.pdf" which guides you how to build/setup the source/image for i.MX board.
After completing the setup, you can find the u-boot source in "<build_directory>/tmp/work/imx6ulevk-poky-linux-gnueabi/u-boot-imx/2017.03-r0/git" path.
And there are different uboot configs files given in this source.
Hope, this will help you.
Let us know if have any concern.
Regards,
Shivani