st-util: open usb can return sl pointer with no target connected.

Check if connected target chip id is known. Some more info regarding
connected chip should be set in chipid to know if debugging / attach
is possible, but not yet.

src/gdbserver/gdb-server.c

@@ -234,7 +234,14 @@ int main(int argc, char** argv) {
     printf("st-util %s\n", STLINK_VERSION);
 
     sl = do_connect(&state);
-    if (sl == NULL) return 1;
+    if (sl == NULL) {
+        return 1;
+    }
+
+    if (sl->chip_id == STLINK_CHIPID_UNKNOWN)  {
+        ELOG("Unsupported Target (Chip ID is %#010x, Core ID is %#010x).\n", sl->chip_id, sl->core_id);
+        return 1;
+    }
 
     connected_stlink = sl;
     signal(SIGINT, &cleanup);
@@ -245,10 +252,7 @@ int main(int argc, char** argv) {
         stlink_reset(sl);
     }
 
-
-    // This is low-level information for debugging, not useful for normal use. 
-    // So: Demoted to a debug meesage. -- REW
-    DLOG("Chip ID is %08x, Core ID is  %08x.\n", sl->chip_id, sl->core_id);
+    DLOG("Chip ID is %#010x, Core ID is %#08x.\n", sl->chip_id, sl->core_id);
 
     sl->verbose=0;
     current_memory_map = make_memory_map(sl);
@@ -1852,7 +1856,8 @@ int serve(stlink_t *sl, st_state_t *st) {
                 stlink_close(sl);
 
                 sl = do_connect(st);
-                if (sl == NULL) cleanup(0);
+                if (sl == NULL || sl->chip_id == STLINK_CHIPID_UNKNOWN)
+                    cleanup(0);
                 connected_stlink = sl;
 
                 if (st->reset) {