Optimize chip select management for SPI bus
This update resolves (possible) race conditions on the SPI bus where ATN asserts (Xbee has frame ready to send) since we last cleared the SPI bus and when we're about to dispatch a frame.

In this case the bus is now claimed and incoming frames processed and cleared before the frame is transmitted.
Fix another possible race condition with incoming packets when transmitting AT command packets.

Fix a bug with interactive sketch where config elements that, when changed, did not cause the Xbee to be updated were also not being updated in EEPROM.