MEGAphone Cellular Modem Communications: SMS Messaging

In recent blog posts I've got the telephony and text messaging software working. What isn't there yet, is making it all talk to the cellular modem -- so that's what we're going to finish here. So let's start by getting some hardware organised that can talk to the cellular modem. We're prototyping using a real MEGA65 mainboard, so we need to tweak a build for that, which has two buffered UARTs…