test build for latency/jitter improvements:
- sei() after time critical phase in servo isr
- vr read does not need atomic block since elements are uint8
- add rud_sw in v2 to reduce servo frame period to 10ms
- move rx in and servo out shared var update out of the pid stage, now they are clocked by servo frame period. need more improvements here..

todo
- alternate i2c and imu libs
- lock free implementation of
test build rx3s v2.
this build does not run the pid stage (since it is suspected that the i2c/imu lib is introducing significant jitter)
build with the right target this time.
= use 16bit timer 1
- for both rx in and servo out
- servo out is channel serialized, may look into concurrent next.
- RX3S_V2_NANO_TIMER1.hex is for the NANO target only.

- uncomment #define USE_SERIAL to dump sensors and sweep the servos on init()
- uncomment #define USE_I2C to use the IMU sensor.
= polling-based i2c access and mpu6050 code
- referenced from multiwii

= use uint16_t instead of int16_t for timer1 math
- overflow handling

uncomment #define USE_I2CDEVLIB to use i2cdevlib and arduino interrupt-based i2c libs
uncomment #define USE_I2CLIGHT to use the lighter weight i2c and mpu6050 code.
= USE_I2CLIGHT
- added itg3205 support
- set i2c clock speed to 400khz

= minor code cleanup
new hex files after merging back to master.
deleted test hex files.
define one device type.