target: Add TARGET_UNAVAILABLE state. (#752)

This is added for future RISC-V changes. The RISC-V debug interface can
explicitly tell a debugger when a hart is unavailable. This is used for
instance when that hart is powered down (or yet to be powered up out of
reset).

Change-Id: I8a062d59eea1e5b3c788281a75159592db024683
Signed-off-by: Tim Newsome <tim@sifive.com>

src/target/target.c

@@ -256,6 +256,7 @@ static const struct jim_nvp nvp_target_state[] = {
 	{ .name = "halted",  .value = TARGET_HALTED },
 	{ .name = "reset",   .value = TARGET_RESET },
 	{ .name = "debug-running", .value = TARGET_DEBUG_RUNNING },
+	{ .name = "unavailable", .value = TARGET_UNAVAILABLE },
 	{ .name = NULL, .value = -1 },
 };
 

src/target/target.h

@@ -56,6 +56,8 @@ struct gdb_fileio_info;
  * not sure how this is used with all the recent changes)
  * TARGET_DEBUG_RUNNING = 4: the target is running, but it is executing code on
  * behalf of the debugger (e.g. algorithm for flashing)
+ * TARGET_UNAVAILABLE = 5: The target is unavailable for some reason. It might
+ * be powered down, for instance.
  *
  * also see: target_state_name();
  */
@@ -66,6 +68,7 @@ enum target_state {
 	TARGET_HALTED = 2,
 	TARGET_RESET = 3,
 	TARGET_DEBUG_RUNNING = 4,
+	TARGET_UNAVAILABLE = 5
 };
 
 enum nvp_assert {