Helo Nagaprasad Vasamsetti,
You can see the available MACHINES on any particular BSP released listed on the Build Configurations section of the Freescale Yocto Project User’s Guide included as part of the BSP documentation.
If you are using the Community BSP this information is available on the Supported Board List available on the Community BSP documentation.
The MACHINE that corresponds to the i.MX6UL EVK in both BSPs is: “imx6ulevk”
I hope this information helps!
Regards,