MFC r262337: Allow the boot device to be choosen by a u-boot env variable.
MFC r262337: Allow the boot device to be choosen by a u-boot env variable.

  If a "loaderdev=<device>" env variable is set and the named device
  exists, it is used.  If the device doesn't exist, fall back to the
  historic "probe" loop that prefers disk devices over network devices.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase