TODO: shared/gatt-server notifications done.
tools/btgatt-server: Introduce btgatt-server.

This patch introduces tools/btgatt-server, which is a command-line tool
for testing and debugging shared/gatt-server.
tools/btgatt-server: Accept incoming connection and initialize server.

This patch adds code that listens for incoming connections and creates a
bt_gatt_server and bt_att structure once a connection is accepted.
IN PROGRESS: Populate DB
tools/btgatt-server: Add command support and the notify command.

This patch adds a command REPL and introduces the "notify" command which
allows sending notifications and indications.
tools/btgatt-client: DO NOT SUBMIT: Added "att" command for testing.