Make the message that informs about bootcode being written to disk less
confusing.

Note there is still no information about 'partcode' being written to disk
(gpart bootcode -p <partcode> <disk>).

Maybe in the future all the messages printed by gpart(8) on success could be
hidden under -v?

PR: bin/150239
Reported by: Roddi <roddi@me.com>
Submitted by: arundel
MFC after: 2 weeks