MFC r257210, r258527: No hardfloat in ubldr, save/restore r9 for api calls.
MFC r261819 & r262242: Actually validate the header of a loaded dtb file.
MFC r262326 (white space) and r262327 (fdt_addr).

This change was supposed to be whitespace only, but a functional change
snuck in, as follows:

  Look for both fdtaddr and fdt_addr env var names.  Grepping the u-boot
  source shows that board vendors seem to be about evenly split on this.
  The added lines were:

    /* Board vendors use both fdtaddr and fdt_addr names.  Grrrr. */

    if (s == NULL)
        s = ub_env_get("fdt_addr");
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.