Tools: ardupilotwaf: chibios: change directory before running make_abin.sh

Any waf build which is done outside of the git checkout will fail to create the abin without this, as it attempts to invoke git.

An example of this is the autotest system

Tools/ardupilotwaf/chibios.py

@@ -81,7 +81,7 @@ class generate_fw(Task.Task):
     run_str='${OBJCOPY} -O binary ${SRC} ${SRC}.bin && \
     python ${UPLOAD_TOOLS}/px_mkfw.py --image ${SRC}.bin \
     --prototype ${BUILDROOT}/apj.prototype > ${TGT} && \
-    ${TOOLS_SCRIPTS}/make_abin.sh ${SRC}.bin ${SRC}.abin'
+    cd ${TOOLS_SCRIPTS} && ./make_abin.sh $OLDPWD/${SRC}.bin $OLDPWD/${SRC}.abin'
     always_run = True
     def keyword(self):
         return "Generating"