Only output logf() to the serial port if LOGF_SERIAL is defined
(in addition to HAVE_SERIAL). Add an option to advanced configure
to set this (defaults to off).
Change-Id: I13d0f5df391e5371f7797f46a3faaf7a6974b9e4
Reviewed-on: http://gerrit.rockbox.org/92
Reviewed-by: Björn Stenberg <bjorn@haxx.se>