Add task_backtrace command

The task_backtrace (aliased as tbt) command allows viewing the backtrace of a task which is currently not running.

init.gdb

@@ -3,3 +3,33 @@ target remote localhost:3333
 define reset
     mon mww 0x40000004 0x80000000
 end
+
+
+# usage: task_backtrace <tskTCB*>
+define task_backtrace
+    set $taskbt_task_ptr = $arg0
+    set $taskbt_stack_ptr = $taskbt_task_ptr->pxTopOfStack
+
+    set $taskbt_frame_offset = 9
+    if ((*(uint32_t*)($taskbt_stack_ptr + 8)) & 0x10 == 0)
+        echo FPU is on\n
+        set $taskbt_frame_offset += 16
+    else
+        echo FPU is off\n
+    end
+
+    set $taskbt_reg_lr = $lr
+    set $taskbt_reg_pc = $pc
+    set $taskbt_reg_sp = $sp
+
+    set $lr = *($taskbt_stack_ptr + $taskbt_frame_offset + 5)
+    set $pc = *($taskbt_stack_ptr + $taskbt_frame_offset + 6)
+    set $sp = $taskbt_stack_ptr + $taskbt_frame_offset + 8
+
+    bt
+
+    set $lr = $taskbt_reg_lr
+    set $pc = $taskbt_reg_pc
+    set $sp = $taskbt_reg_sp
+end
+alias tbt = task_backtrace