Going back to the machine configuration file, we now need to look into one of the include files we mentioned before:

include conf/machine/include/imx-base.inc   

Inside that file, the U-Boot preferred provider is defined:

PREFERRED_PROVIDER_u-boot ??= "u-boot-fslc" 
PREFERRED_PROVIDER_virtual/bootloader ??= "u-boot-fslc" 

So u-boot-fslc is the U-Boot recipe we are looking for.

Using bitbake-layers as before, we can find its containing layer and version:

$ bitbake-layers show-recipes u-boot-fslc 
=== Matching recipes: === 
u-boot-fslc: 
  meta-freescale       v2017.11+gitAUTOINC+ca0c3f3fac 

The Wandboard machine is using u-boot-fslc on its v2017.11 version. As we can see, the package version defined in the recipe also contains the specific SHA-1 being built.