Bluetooth: BR: Shell: Add L2CAP basic function commander

Add a sub-command set `l2cap` for command set `br`.

Move command `l2cap-register` to sub-command set. And rename it to
`register`.

Add command `connect`, `disconnect`, and `send` for command set
`l2cap`.

Remove original net buffer pool from `data_pool` to `data_rx_pool`.
Add a net buffer pool `data_tx_pool` for command `send`.

Do not wait anymore if no net buffer can be allocated from
`data_rx_pool`.

Dump all received data in L2CAP data received callback.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>