Track gyro using a DCM.

This brings in an implementation of some general matrix manipulation routines,
not all of which are used at teh moment.

Output the DCM on the UART at 50Hz for the host to display it usefully.

uart.h

@@ -7,6 +7,7 @@ void init_uart(void);
 void putch(char c);
 void putstr(char *s);
 void putint(unsigned int n);
+void putint_s(int n);
 void puthex(unsigned int n);
 bool getch(char *c);