By default, some of the machine configurations recommend packages for the WiFi/BT firmware, provided by linux-firmware-rpidistro. This package includes some firmware blobs under the Synaptics license which could carry a legal risk: one of the clauses can be (at least theoretically) used as a killswitch. This was reported in the upstream repository.
You can find the full license text body in the content of the above mentioned package.
Due to the above, the build system will only allow this recipe to be built if the user acknowledges this risk by adding the following configuration:
LICENSE_FLAGS_ACCEPTED = "synaptics-killswitch"
You can provide this configuration as part of your local.conf, distro.conf, etc.