[FR] expose `write(_, buf: &[u8])` when `set_print_channel()` used

[ildar]

the set_print_channel() consumes the channel hence write(channel, buf: &[u8]) can't be used. And rtt_print() doesn't accept &[u8].
The situation when I need to write [u8] is trivial: to echo bytes I receive with read() from down channel.

[mvirkkunen]

If you want to send actual binary data, I think it's better to write directly to the channel. The printing facility is really only meant for printing text. If it's actually text, you can use core::str::from_utf8 to convert it to an str for printing.