Hello Liang Feng,
The L4.9.88_2.0.0 BSP Release does also support the i.MX6ULL EVK as shown on the Build Configurations section of the i.MX Yocto Project User's Guide (available as part of the BSP documentation bundle).
The MACHINE would be imx6ulevk if you are using the i.MX6UL EVK
However, the sfl-image-qt5-validation-imx image cannot be built for the i.MX6UL EVK as Qt5 requires a GPU for Graphic Hardware Acceleration.
I hope this information helps!
Regards,