rtos: add instructions and helper code to make FreeRTOS work again

Run-time tested with FreeRTOS V8.1.2 (current version).

For the time being I propose this way of dealing with RTOSes that do
not export necessary information on their own.

I also suggest implementing a similar scheme for ChibiOS, exporting
the necessary struct fields' offsets via an OpenOCD-specific helper.

Change-Id: Iacf8b88004d62206215fe80011fd7592438446a3
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2347
Tested-by: jenkins

contrib/rtos-helpers/FreeRTOS-openocd.c

@@ -0,0 +1,20 @@
+/*
+ * Since at least FreeRTOS V7.5.3 uxTopUsedPriority is no longer
+ * present in the kernel, so it has to be supplied by other means for
+ * OpenOCD's threads awareness.
+ *
+ * Add this file to your project, and, if you're using --gc-sections,
+ * ``--undefined=uxTopUsedPriority'' (or
+ * ``-Wl,--undefined=uxTopUsedPriority'' when using gcc for final
+ * linking) to your LDFLAGS; same with all the other symbols you need.
+ */
+
+#include "FreeRTOS.h"
+
+#ifdef __GNUC__
+#define USED __attribute__((used))
+#else
+#define USED
+#endif
+
+const int USED uxTopUsedPriority = configMAX_PRIORITIES;