Bluetooth: shell: Split L2CAP commands

This splits L2CAP commands under 'bt' to 'l2cap' removing 'l2cap-'
prefix from the commands:

l2cap - Bluetooth L2CAP shell commands
Options:
  -h, --help  :Show command help.
Subcommands:
  connect     :<psm>
  disconnect  :[none]
  metrics     :<value on, off>
  recv        :[delay (in miliseconds)
  register    :<psm> [sec_level]
  send        :<number of packets>

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