Bluetooth: shell: Split RFCOMM commands

This splits RFCOMM command under 'br' to 'rfcomm' removing 'rfcomm-'
prefix from commands:

rfcomm - Bluetooth RFCOMM shell commands
Options:
  -h, --help  :Show command help.
Subcommands:
  register    :<channel>
  connect     :<channel>
  disconnect  :[none]
  send        :<number of packets>

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>