Shell: kernel: Add runtime log filtering command

Use `kernel log-level modulename severity`
Also enable it for the Bluetooth Shell.

Then one can compile-in a lot of BT modules like so:

CONFIG_BT_DEBUG_HCI_CORE=y
CONFIG_BT_DEBUG_L2CAP=y
CONFIG_BT_DEBUG_ATT=y
CONFIG_BT_DEBUG_GATT=y

And at runtime select only, e.g. GATT

kernel log-level bt_hci_core 0
kernel log-level bt_l2cap 0
kernel log-level bt_att 0

And then re-enable L2CAP if needed later

kernel log-level bt_l2cap 4

And so on..

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>