...to flesh out the explanations a bit and link back to OVERRIDES:

  MACHINEOVERRIDES:
    A colon-separated list of overrides that apply to the current machine. By
    default, this list includes the value of MACHINE.
  
    You can extend MACHINEOVERRIDES to add extra overrides that should apply to a
    machine. For example, all machines emulated in QEMU (e.g. qemuarm, qemux86,
    and so forth) include a file named meta/conf/machine/include/qemu.inc that
    prepends the following override to MACHINEOVERRIDES:
  
      MACHINEOVERRIDES =. "qemuall:"
  
    This override allows variables to be overriden for all machines emulated in
    QEMU, like in the following example from the connman-conf recipe:
  
      SRC_URI_append_qemuall = "file://wired.config \
                                file://wired-setup \
                               "
  
    The underlying mechanism behind MACHINEOVERRIDES is simply that it is
    included in the default value of [OVERRIDES].


Cheers,
Ulf