Reorder all instances of accel-gyro to gyro-accel.

Gyros are more important.
Make heartbeat LED pattern for aerial_v3 fancier.
Disable TIM1 and TIM3 for aerial_v3.
Enable USART3 for aerial_v3.
Print some test messages on USART1 and USART3.
Reorder quadrotor motors per convention.
Fix gyro-accel ordering.

Forgot this one.
Initialize SPI1 pins.