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.

Makefile

@@ -3,7 +3,7 @@
 NAME=quad
 
 SSRCS=crt0.s
-CSRCS=main.c i2c.c wmp.c timer.c interrupt.c uart.c event.c
+CSRCS=main.c i2c.c wmp.c timer.c interrupt.c uart.c event.c matrix.c dcm.c
 
 COPTIM?=-O1
 CFLAGS=-march=armv4t -msoft-float $(COPTIM) -Wall -Werror -Wextra