Revert "target: remove unused working area 'user' field"

This reverts commit 63a23e6fc862b94f00e0833ab474bd02901a019f, which
causes problems with various flash drivers (i.e. LPC1xxx/LPC2xxx)
due to working area handling.

Change-Id: I291ff2ac4f1eef7ecd553d0eb8895b790beb38c3
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/872
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/target.c

@@ -1405,6 +1405,7 @@ static void target_split_working_area(struct working_area *area, uint32_t size)
 		new_wa->size = area->size - size;
 		new_wa->address = area->address + size;
 		new_wa->backup = NULL;
+		new_wa->user = NULL;
 		new_wa->free = true;
 
 		area->next = new_wa;
@@ -1493,6 +1494,7 @@ int target_alloc_working_area_try(struct target *target, uint32_t size, struct w
 			new_wa->size = target->working_area_size & ~3UL; /* 4-byte align */
 			new_wa->address = target->working_area;
 			new_wa->backup = NULL;
+			new_wa->user = NULL;
 			new_wa->free = true;
 		}
 
@@ -1536,6 +1538,9 @@ int target_alloc_working_area_try(struct target *target, uint32_t size, struct w
 	c->free = false;
 	*area = c;
 
+	/* user pointer */
+	c->user = area;
+
 	print_wa_layout(target);
 
 	return ERROR_OK;
@@ -1586,6 +1591,13 @@ static int target_free_working_area_restore(struct target *target, struct workin
 	LOG_DEBUG("freed %"PRIu32" bytes of working area at address 0x%08"PRIx32,
 			area->size, area->address);
 
+	/* mark user pointer invalid */
+	/* TODO: Is this really safe? It points to some previous caller's memory.
+	 * How could we know that the area pointer is still in that place and not
+	 * some other vital data? What's the purpose of this, anyway? */
+	*area->user = NULL;
+	area->user = NULL;
+
 	target_merge_working_areas(target);
 
 	print_wa_layout(target);
@@ -1613,6 +1625,8 @@ static void target_free_all_working_areas_restore(struct target *target, int res
 			if (restore)
 				target_restore_working_area(target, c);
 			c->free = true;
+			*c->user = NULL; /* Same as above */
+			c->user = NULL;
 		}
 		c = c->next;
 	}

src/target/target.h

@@ -95,6 +95,7 @@ struct working_area {
 	uint32_t size;
 	bool free;
 	uint8_t *backup;
+	struct working_area **user;
 	struct working_area *next;
 };