Bluetooth: L2CAP: Add initial public API

This split L2CAP API so that server API is available to applications
while the rest of the API is keep internal to the stack.

Change-Id: I031926ff906ce100684fba0947b2e9eb2c8fcaeb
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>