UART instability fix with high interrupt load (commaai#283)

* Fixed UART overrun error

* Added stability test scripts

* Refactored UART code. ESP/GPS now uses DMA in circular mode to directly write into the ring buffer, saving a bunch of interrupts and potential race conditions.

* Changed stability test to use amount of bytes instead of amount of non-zero messages, since the ring buffer pointer is only updated on half or full DMA transfer or on line idle (e.g. no more 1 char messages from the gps)

* Increase data limit. It's too low sometimes in normal operation

* Forgot to set write pointer in ring buffer

* Increased test limit even more

board/config.h

@@ -2,6 +2,7 @@
 #define PANDA_CONFIG_H
 
 //#define DEBUG
+//#define DEBUG_UART
 //#define DEBUG_USB
 //#define DEBUG_SPI