Add bthost, a simple BLE GATT server.

This is accessible via Unix socket.
It only has a couple of interesting features:

 * Built in support for large blob attributes (>512 octets)
 * Attribute caching (avoid frame-level IO for IPC clients)

Some string utilies are taken from modp_b64 and Chromium base.

Bug: 21076037
Change-Id: I6a29959159de76f8dd68d6bbaabe2100daabb6fa