x86: upgrade: make code comment appear as log lines

x86: upgrade: make code comment appear as log lines

The comment content can be useful for readers of both the log and code

Previously when dd command "records in/out" messages are not filtered
like now with get_image_dd, it's not clear that these messages are for
extracting boot sectors.  E.g.

Before

  == upgrade: Reading partition table from bootdisk...
  37+26 records in
  37+26 records out
  == upgrade: Reading partition table from image...

After

  == upgrade: Reading partition table from bootdisk...
  == upgrade: Extract boot sector from the image
  37+26 records in
  37+26 records out
  == upgrade: Reading partition table from image...

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>