Update the usr.bin/mkimg golden test output files after ^/head@r319125

^/head@r319125 changed the location of the backup pmbr, requiring the
output files to be regenerated, since they're binary disk dumps.

The output files were regenerated with "make rebase"--fixed in
^/head@r319294.

MFC with: r319125, r319294
PR: 219673
Sponsored by: Dell EMC Isilon