added (untested) flag to disable tricopter servo updates when unarmed.
tri_unarmed_servo to 1 (default) always updates tri servo whether armed or not.
tri_unarmed_servo to 0 will only send servo signal to tail servo when armed.

git-svn-id: https://afrodevices.googlecode.com/svn/trunk/baseflight@392 7c89a4a9-59b9-e629-4cfe-3a2d53b20e61
Totally rework software serial to provide tx at the same time as rx using only one timer.