tests/migration: Add source to PC boot block

tests/migration: Add source to PC boot block

The boot block used in the migration test is currently only
shipped as a hex (with the source in the git commit message of ea0c6d62),
change this to actually include the source.

A script is added to rebuild the header but the expectation is that
the generated hex is shipped as well as the .s, so that
there's no requirement to have just the right assembler etc.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20180213100606[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
  Removed blank line at end of script