[bt][common] Unified logging primitive for bt-host

Added printf-style logging utility that can work with both syslog
(fx_logger_log) and DDK debug log (zxlogf/driver_printf).

NET-357
Test: 1. Test that bt-host log messages can be configured via kernel
         command-line by passing variants of
	 "driver.bthost.log=+info,+trace,+spew".
      2. Test that bt-host-unittests prints to stdout using syslog
         messages.

Change-Id: Iab5d88e5f55f8b8616b4fc5c2661a355d0c12d18
14 files changed