target: add debug_reason_name()

Provide and use debug_reason_name() instead of expecting targets
to call Jim_Nvp_value2name_simple().  Less dependency on Jim, and
the code becomes more clear too.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/target.h

@@ -91,8 +91,6 @@ enum target_debug_reason
 	DBG_REASON_UNDEFINED = 6
 };
 
-extern const Jim_Nvp nvp_target_debug_reason[];
-
 enum target_endianess
 {
 	TARGET_ENDIAN_UNKNOWN = 0,
@@ -165,6 +163,8 @@ static inline const char *target_name(struct target *target)
 	return target->cmd_name;
 }
 
+const char *debug_reason_name(struct target *t);
+
 enum target_event
 {
 	/* LD historical names